This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes.
For the nitriding process H2 gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding see for instance. Boriding also called boronizing, is the process by which boron is introduced to a metal or alloy. It is a type of surface hardening. In this process boron atoms are diffused into the surface of a metal component. The resulting surface contains metal borides, such as iron borides, nickel borides, and cobalt borides, As pure materials, these borides have extremely high hardness and wear resistance.
Their favorable properties are manifested even when they are a small fraction of the bulk solid. Boronized metal parts are extremely wear resistant and will often last two to five times longer than components treated with conventional heat treatments such as hardening, carburizing, nitriding, nitrocarburizing or induction hardening. Most borided steel surfaces will have iron boride layer hardnesses ranging from HV.
Nickel-based superalloys such as Inconel and Hastalloys will typically have nickel boride layer hardnesses of HV. Typical boriding mixture consists of boron carbide powder diluted with other refractory materials such as alumina or silicon carbide. Potassium tetrafluoroborate KBF4 is used as a flux. The process converts some of the Fe to iron boride, consisting of two phases: FeB concentrated near the surface, and diiron boride Fe2B. It is possible to combine with other heat treatments such as carburizing, hardening or induction hardening to create deeper wear layers or high core hardness.
It is often used on steel, but is applicable to a variety of alloys and cermet materials. A wide range of materials suitable for treatment including plain carbon steels, alloy steels, tool steels, nickel-based super alloys, cobalt alloys, and stellite. The following materials not compatible with boronizing: stainless steels, nitrogen, aluminum or silicon containing grades.
Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it In some cases it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process. Carburization of steel involves a heat treatment of the metallic surface using a source of carbon.
Carburization can be used to increase the surface hardness of low carbon steel. Early carburization used a direct application of charcoal packed onto the metal initially referred to as case hardening , but modern techniques apply carbon-bearing gases or plasmas such as carbon dioxide or methane.
The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber. Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics such as wear and corrosion resistance, hardness and load-bearing capacity, in addition to quality-based variables of various metals, notably stainless steels.
The process is used as it is environmentally friendly in comparison to gaseous or solid carburizing. It also provides an even treatment of components with complex geometry the plasma can penetrate into holes and tight gaps , making it very flexible in terms of component treatment. The process of carburization works via the implantation of carbon atoms into the surface layers of a metal.
As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution dissolved within the metal crystalline matrix — this normally occurs at lower temperatures or react with the host metal to form ceramic carbides normally at higher temperatures, due to the higher mobility of the host metal's atoms. Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion.
However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst impairing its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to Engineers using plasma carburization must decide which of the two mechanisms matches their needs. In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame.
It is often used to anneal metal, making it more malleable and flexible during the welding process. A main goal when producing carbonized workpieces is to ensure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire.
In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickel-chromium alloy and sealed at all openings with fire clay. Since the process involves chemical decomposition of precursor gas and physical evaporation of metal bulk, it is named as hybrid physical-chemical vapor deposition.
Other MgB2 deposition technologies either have a reducedsuperconducting transition temperature and poor crystallinity, or require ex situ annealing in Mg vapor. The surfaces of these MgB2 films are rough and non-stoichiometric. Instead, HPCVD system can grow high- quality in situ pure MgB2 films with smooth surfaces, which are required to make reproducible uniformJosephson junctions, the fundamental element of superconducting circuits.
During the growth process of magnesium diboride thin films by HPCVD, the carrier gas is purified hydrogen gas H2 at a pressure of about Torr. This H2environment prevents oxidation during the deposition. Bulk pure Mg pieces are placed next to the substrate on the top of the susceptor. Diborane B2H6 is used as theboron source.
MgB2 films starts to grow when The film growth stops when the boron precursor gas is switched off. The HPCVD system usually consists of a water-cooled reactor chamber, gas inlet and flow control system, pressure maintenance system, temperature control system and gas exhaust and cleaning system. The substrate and solid metal source sit on the same susceptor and are heated up inductively or resistively at the same time.
Above certain temperature, the bulk metal source melts and generates a high vapor pressure in the vicinity of the substrate. Then the precursor gas is introduced into the chamber and decomposes around the substrate at high temperature. The atoms from the decomposed precursor gas react with the metal vapor, forming thin films on thesubstrate. The deposition ends when the precursor gas is switched off.
The main drawback of single heater setup is the metal source temperature and the substrate temperature cannot be controlled independently. Whenever the substrate temperature is changed, the metal vapor pressure changes as well, limiting the ranges of In the two-heater HPCVD arrangement, the metal source and substrate are heated up by two separate heaters. Most coatings have high temperature and good impact strength, excellent abrasion resistance and are so durable that protective topcoats are almost never necessary.
Application As mentioned previously, PVD coatings are generally used to improve hardness, wear resistance and oxidation resistance. The process is often used in the semiconductor industry to produce thin films.
Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber. Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline,amorphous, and epitaxial. These materials include: silicon SiO2, germanium, carbide, nitride, oxynitride , carbon fiber,nanofibers, nanotubes, diamond and graphene , fluorocarbons, filaments, tungsten, titaniu m nitride and various high-k dielectrics.
These processes generally differ in the means by which chemical reactions are initiated. This technique is suitable for use with non-volatile precursors. Liquid solutions are injected in a vaporization chamber towards injectors typically car injectors. The precursor vapors are then transported to the substrate as in classical CVD. This technique is suitable for use on liquid or solid precursors. High growth rates can be reached using this technique. PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors.
The lower temperatures also allow for the deposition of organic coatings, such as plasma polymers, that have been used for nanoparticle surface functionalization. Removing the wafer from the plasma region allows processing temperatures down to room temperature. See Atomic layer epitaxy. Heating only the substrate rather than the gas or chamber walls helps reduce unwanted gas-phase reactions that can lead to particle formation.
It is similar to plasma processing, given that plasmas are strong emitters of UV radiation. Use of Chemical Vapor Deposition CVD: CVD is commonly used to deposit conformal films and augment substrate surfaces in ways that more traditional surface modification techniques are not capable of. CVD is extremely useful in the process of atomic layer deposition at depositing extremely thin layers of material. A variety of applications for such films exist.
Gallium arsenide is used in some integrated circuits ICs and photovoltaic devices. Amorphous polysilicon is used in photovoltaic devices. Certain carbides and nitridesconfer wear-resistance. Polymerization by CVD, perhaps the most versatile of all applications, allows for super-thin coatings which possess some very desirable qualities, such as lubricity, hydrophobicity and weather-resistance to name a few.
CVD of metal- organic frameworks, a class of crystalline nanoporous materials, has recently been demonstrated. Applications for these films are anticipated in gas sensing and low-k dielectrics. This process is used to change the physical, chemical, or electrical properties of the solid. Ion implantation is used in semiconductor device fabrication and in metal finishing, as well as various applications inmaterials science research. The ions alter the elemental composition of the target if the ions differ in composition from the target , stopping in the target and staying there. They also cause many chemical and physical changes in the target by transferring their energy and momentum to the electrons and atomic nuclei of the target material.
This causes a structural change, in that the crystal structure of the Because the ions have masses comparable to those of the target atoms, they knock the target atoms out of place more than electron beams do. If the ion energy is sufficiently high usually tens of MeV to overcome the coulomb barrier, there can even be a small amount of nuclear transmutation. Ion implantation equipment typically consists of an ion source, where ions of the desired element are produced, anaccelerator, where the ions are electrostatically accelerated to a high energy, and a target chamber, where the ions impinge on a target, which is the material to be implanted.
Thus ion implantation is a special case of particle radiation. Each ion is typically a single atom or molecule, and thus the actual amount of material implanted in the target is the integral over time of the ion current. This amount is called the dose. The currents supplied by implanters are Therefore, ion implantation finds application in cases where the amount of chemical change required is small.
Typical ion energies are in the range of 10 to keV 1, to 80, aJ. Energies in the range 1 to 10 keV to 1, aJ can be used, but result in a penetration of only a few nanometers or less. Energies lower than this result in very little damage to the target, and fall under the designation ion beam deposition. Higher energies can also be used: accelerators capable of 5 MeV , aJ are common. However, there is often great structural damage to the target, and because the depth distribution is broad Bragg peak , the net composition change at any point in the target will be small.
The energy of the ions, as well as the ion species and the composition of the target determine the depth of penetration of the ions in the solid: A monoenergetic ion beam will generally have a broad depth distribution. The average penetration depth is called the range of the ions. Under typical circumstances ion ranges will be between 10 nanometers and 1 micrometer.
Thus, ion implantation is especially useful in cases where the chemical or structural change is desired to be near the surface of the target. Ions gradually lose their energy as they travel through the solid, both from occasional collisions with target atoms which cause abrupt energy transfers and from a mild drag from overlap of electron orbitals, which is a continuous process.
The loss of ion energy in the target is called stopping and can be simulated with the binary collision approximation method. All varieties of ion implantation beamline designs contain certain general groups of functional components see image. The first major segment of an ion beamline includes a device known as an ion source to generate the ion species.
The source is closely coupled to biased electrodes for extraction of the ions into the beamline and most often to some means of selecting a particular ion species for transport into the main accelerator section. The "mass" selection is often accompanied by passage of the extracted ion beam through a magnetic field region with an exit If the target surface is larger than the ion beam diameter and a uniform distribution of implanted dose is desired over the target surface, then some combination of beam scanning and wafer motion is used.
Finally, the implanted surface is coupled with some method for collecting the accumulated charge of the implanted ions so that the delivered dose can be measured in a continuous fashion and the implant process stopped at the desired dose level. Application in semiconductor device fabrication Doping : The introduction of dopants in a semiconductor is the most common application of ion implantation.
Dopant ions such as boron, phosphorus or arsenic are generally created from a gas source, so that the purity of the source can be very high. These gases tend to be very hazardous. When implanted in a semiconductor, each dopant atom can create a charge carrier in the semiconductor after annealing. A hole can be created for a p-type dopant, and an electron for an n-type dopant. This modifies the conductivity of the semiconductor in its vicinity. Ion implantation was developed as a method of producing the p-n junction of photovoltaic devices in the late s and early s, along with the use of pulsed-electron beam for rapid annealing, although it has not to date been used for commercial production.
Engineering the Surface | | Industrial Heating
Silicon on insulator : One prominent method for preparing silicon on insulator SOI substrates from conventional silicon substrates is the SIMOX separation by implantation of oxygen process, wherein a buried high dose oxygen implant is converted to silicon oxide by a high temperature annealing process. Mesotaxy : Mesotaxy is the term for the growth of a crystallographically matching phase underneath the surface of the host crystal compare to epitaxy, which is the growth of the matching phase on the surface of a substrate.
In this process, ions are implanted at a high enough energy and dose into The crystal orientation of the layer can be engineered to match that of the target, even though the exact crystal structure and lattice constant may be very different. For example, after the implantation of nickel ions into a silicon wafer, a layer of nickel silicide can be grown in which the crystal orientation of the silicide matches that of the silicon.
Application in metal finishing Tool steel toughening : Nitrogen or other ions can be implanted into a tool steel target drill bits, for example. The structural change caused by the implantation produces a surface compression in the steel, which prevents crack propagation and thus makes the material more resistant to fracture. The chemical change can also make the tool more resistant to corrosion.
Surface finishing : In some applications, for example prosthetic devices such as artificial joints, it is desired to have surfaces very resistant to both chemical corrosion and wear due to friction. Ion implantation is used in such cases to engineer the surfaces of such devices for more reliable performance. As in the case of tool steels, the surface modification caused by ion implantation includes both a surface compression which prevents crack propagation and an alloying of the surface to make it more chemically resistant to corrosion. Other applications Ion beam mixing : Ion implantation can be used to achieve ion beam mixing, i.
This may be useful for achieving graded interfaces or strengthening adhesion between layers of immiscible materials. Problems with ion implantation Vacancies are crystal lattice points unoccupied by an atom: in this case the ion collides with a target atom, resulting in transfer of a significant amount of energy to the target atom such that it leaves its crystal site.
This target atom then itself becomes a projectile in the solid, and can cause successive collision events. Interstitials result when such atoms or the original ion itself come to rest in the solid, but find no vacant space in the lattice to reside. These point defects can migrate and cluster with each other, resulting in dislocation loops and other defects.
Damage recovery : Because ion implantation causes damage to the crystal structure of the target which is often unwanted, ion implantation processing is often followed by a thermal annealing. This can be referred to as damage recovery. Amorphization : The amount of crystallographic damage can be enough to completely amorphize the surface of the target: i. In some cases, complete amorphization of a target is preferable to a highly defective crystal: An amorphized film can be regrown at a lower temperature than required to anneal a highly damaged crystal.
Sputtering : Some of the collision events result in atoms being ejected sputtered from the surface, and thus ion implantation will slowly etch away a surface. The effect is only appreciable for very large doses. If there is a crystallographic structure to the target, and especially in semiconductor substrates where the crystal structure is more open, particular crystallographic directions offer much lower stopping than other directions. This effect is called ion channelling, and, like all the channelling effects, is highly nonlinear, with small variations from perfect orientation For this reason, most implantation is carried out a few degrees off-axis, where tiny alignment errors will have more predictable effects.
Ion channelling can be used directly in Rutherford backscattering and related techniques as an analytical method to determine the amount and depth profile of damage in crystalline thin film materials. Usually a Kaufman source, like that used in IBS, supplies the secondary beam. IAD can be used to deposit carbon in diamond-like form on a substrate. Any carbon atoms landing on the substrate which fail to bond properly in the diamond crystal lattice will be knocked off by the secondary beam. NASA used this technique to experiment with depositing diamond films on turbine blades in the s.
IAS is used in other important industrial applications such as creating tetrahedral amorphous carbon surface coatings on hard disk platters and hard transition metal nitride coatings on medical implants. Sputter deposition is a physical vapor deposition PVD method of thin film deposition by sputtering.
This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Besides providing independent control of parameters such as ionenergy, temperature and arrival rate of atomic species during deposition, this technique is especially useful to create a gradual transition between the substratematerial and the deposited film, and for depositing films with less built- in strain than is possible by other techniques.
These two properties can result in films with a much more durable bond to the substrate. Experience has shown that some meta- stable compounds like cubic boron nitride c-BN , can only be formed in thin films when bombarded with energetic ions during the deposition process. The process involves bombarding layered samples with doses of ion radiation in order to promote mixing at the interface, and generally serves as a means of preparing electrical junctions, especially between non-equilibrium or metastable alloys and intermetallic compounds.
Ion implantation equipment can be used to achieve ion beam mixing. The unique effects that stem from ion beam mixing are primarily a result of ballistic effects; that is, impinging ions have high kinetic energies that are transferred to target atoms on collision. Ion When accelerated, such ion energies are sufficiently high to break intra- and especially inter-molecular bonds, and initiate relocations within an atomic lattice. The sequence of collisions is known as a collision cascade.
During this ballistic process, energies of impinging ions displace atoms and electrons of the target material several lattice sites away, resulting in relocations there and interface mixing at the boundary layer. Note that energies must be sufficiently high in order for the lattice rearrangements to be permanent rather than manifesting as mere vibrational responses to the impinging radiation, i. If energies are kept sufficiently high in these nuclear collisions, then, compared to traditional high-dose implantation processes, ballistic ion implantation produces higher intrafilm alloy concentrations at lower doses of irradiation compared to conventional implantation processes.
The degree of mixing of a film scales with the ion mass, with the intensity of any given incident ion beam, and with the duration of the impingement of the ion beam on a target. The amount of mixing is proportional to the square roots of time, mass and ion dose. This temperature-dependence is a manifestation of incident ion beams effectively imparting the target species-dependent activation energy to the barrier layer.
In recoil mixing, atoms are relocated by single collision events. Recoil mixing is predominately seen at large angles as a result of soft collisions, with the number of atoms undergoing recoil implantation varying linearly with ion dose. Recoil implantation, however, is not the dominant process in ion beam mixing. Most relocated atoms are part of a collision cascade in which recoiled atoms initiate a series of lower energy lattice displacements, which is referred to as cascade mixing. Ion beam mixing can be further enhanced by heat spike effects Ion mixing IM is essentially similar in result to interdiffusion, hence most models of ion mixing involve an effective diffusion coefficient that is used to characterize thickness of the reacted layer as a function of ion beam implantation over a period of time.
Thermodynamic effects are also not considered in this basic interdiffusion equation, but can be modeled by equations that consider the enthalpies of mixing and the molar fractions of the target species, and one can thereby develop a thermodynamic effective diffusion coefficient reflecting temperature effects which become pronounced at high temperatures. Advantages and disadvantages Advantages of ion beam mixing as a means of synthesis over traditional modes of implantation include the process' ability to produce materials with high soluteconcentrations using lower amounts of irradiation, and better control of band gap variation and diffusion between layers.
The cost of IM is also less prohibitive than that of other modes of film preparation on substrates, such as chemical vapor deposition CVD and molecular beam epitaxy MBE. Disadvantages include the inability to completely direct and control lattice displacements initiated in the process, which can result in an undesirable degree of disorder in ion mixed samples, rendering them unsuitable for applications in which precise lattice orderings are paramount.
Ion beams cannot be perfectly directed, nor the collision cascade controlled, once IM effects propagate, which can result in leaking, electron diffraction, radiation enhanced diffusion RED , chemical migration and mismatch. Additionally, all ion mixed samples must be annealed. These coatings have extremely dense, crack- free and non-porous microstructures.
Laser coatings show also excellent metallurgical bonding to the base material, have uniform composition and coating thickness. Laser coating produces also very low dilution and low heat input to the component. Laser coating of new components gives them surfaces with high resistance against wear, corrosion and high temperatures.
Besides new manufacturing, the process has shown its importance also in maintenance and repair of worn components, often resulting in component performances superior to those of uncoated ones. Industrial use of laser coatings is expected to increase markedly during the following next years. Results of friction and wear tests showed that the friction coefficient of PEO coating against steel was as low as 0.
The investigations of structure, composition, mechanical and tribological properties of PEO coatings formed on AM60B Mg alloy in silicate and phosphate electrolyte have been done by Jun Liang et al. The coating formed in silicate electrolyte is composed of periclase MgO and forsterite Mg 2 SiO 4 phases while MgO and a little of spinel MgAl 2 O 4 are the main phases of the coating formed in phosphate electrolyte. Therefore, the coating formed in silicate electrolyte exhibits a higher microhardness than that formed in phosphate electrolyte.
The friction and wear properties of the PEO coatings were evaluated on a reciprocal-sliding UMT-2MT tribometer in dry sliding conditions under a load of 2 N, using Si 3 N 4 ball as counterpart material, with a siding speed of 0. The wear life of PEO coatings formed in two different electrolytes was compared with the thin coatings and results showed that the wear life of coating formed in silicate electrolyte is about four times as long as that of coating formed in phosphate electrolyte.
The uncoated Mg alloy has a friction coefficient of about 0. While for both the oxide coatings, the friction coefficients are in the range of 0. These evidences demonstrate that the PEO coatings formed on Mg alloy in both electrolytes have greatly enhanced the wear resistance but exhibit higher friction coefficients compared with the uncoated Mg alloy.
Furthermore, the oxide coating formed in silicate electrolyte has a higher friction coefficient but exhibit a better wear resistance than that formed in phosphate electrolyte. It also suggests that the structure and phase composition of coatings are indeed the dominant factors which influence the mechanical property and friction and wear behaviors of PEO coatings. Bala Srinivasan et al. For the uncoated Mg alloy, the friction coefficients were fluctuating in the range of 0. Moreover, the friction coefficient showed lower value with an increase in load 0. The results indicated that the thickness of coatings played a crucial role in enhancing the wear resistance.
At higher initial stress levels, the deformation of the substrate causes the cracking and flaking-off of the coating, especially when it is thin. Under such circumstances the increased thickness of PEO coating provided a better load bearing capacity, thus resulting in a superior wear resistance. The tribological properties were evaluated by a pin on disc test machine, with a sliding distance of 1 km and a constant linear speed of Friction and wear tests showed that the friction coefficient changed along the tests, and the weight loss depended on both the thickness of the coatings and the loads applied during the test.
The wear mechanisms were suggested to be adhesion and abrasion by hard particles. Chen Fei et al. The tribological behaviors of unpolished coating, polished coating and untreated Ti6Al4V alloy were evaluated on a ball-on-disk tribometer under the dry sliding conditions, using balls of SAE steel as counterpart materials, with normal load of N, rotation speed of rpm, sliding speed of 0. For the untreated Ti6Al4V alloy, the long-term friction coefficient is about 0.
For the unpolished PEO coating, the friction coefficient exhibited a high value of about 0. The porous surface of the unpolished PEO coatings is very rough due to the scraggy ceramic products. Unlike sliding that usually leads to plastic shearing in materials, the impact caused by the ceramic asperities on the surface results in catastrophic failure, such as cracking and crushing of the contact regions, which leads to faster material removal and the production of the sharp ceramic debris fragments.
In contrast, for the polished coating, the friction coefficient exhibited a relatively low and stable value, almost remaining constant at 0. Therefore, the cracking and crushing of prominent ceramic regions due to great vibrations were eliminated. Results showed that the worn surface was relatively smooth, accompanied with fine debris embedded in the edges of contact regions. The good antifriction properties are attributed to the microstructure of the coatings which are mainly composed of rutile and anatase TiO 2. TiO 2, especially the rutile-type, is known as a potentially low friction and wear reducing material.
Jun Tian et al. The results of structural and phase composition analysis showed that the PEO coatings on Al alloys showed two distinct layers, i. Therefore, with the increasing of the coating thickness, the antiwear life of the outer layer becomes smaller than that of the inner layer. The antiwear life of the polished coating reached m at a speed of 1.
The aforementioned studies revealed that the PEO ceramic coatings can sharply increase the wear resistance and decrease the wear rate, compared to the uncoated substrates. However, the PEO coatings normally exhibit higher friction coefficients which can cause not only the wear of sliders, but also the wear damage of counterpart materials in many tribological applications. Thus, it is necessary to fabricate the PEO coatings with both good wear resistance and low friction coefficient.
In order to further improve the tribological properties of the PEO-treated lightweight metals, many attempts to reduce the friction coefficient of the PEO coatings have been made. Herein, three main developments in improvement of tribological properties of PEO coatings are reviewed, which can be categorized as 1 liquid lubrication, 2 duplex coatings and 3 composite coatings. As there are many micropores, microcracks and dimples on the surface of the PEO coatings [ 17 ], these pores, cracks and dimples can act as reservoirs for oil lubricants, which may result in a positive effect to the tribological performance of PEO coatings under boundary-lubricated conditions.
Studies on the wear resistance of PEO coatings on Al alloy under oil-lubricated condition were done by Tongbo Wei et al. The friction and wear tests were carried on an MRH-3 ring-on-block tester, at a ring linear speed of 2. Commercial lubricating oil was used as the lubricating medium. Friction and wear test showed that the friction coefficient of polished coatings was within 0. The polished coatings showed excellent wear-resistance in oil-lubricated sliding against steel and Al 2 O 3 ceramic ring and can endure a sliding distance as large as Fei Zhou et al. The wear mechanism of the PEO coatings changed from abrasive wear in air to mix wear in water, and finally became microploughing wear in oil.
Zhu et al. It was found that the friction coefficient of the sealed PEO coating under all test parameters were greatly lower than that of the PEO coating. At the same time, there was a longer stage with low friction coefficient that can be observed in the friction coefficient curves for all test conditions of the sealed PEO coating.
It was clear that the sealed PEO coating presented an obvious lubricating action during the fretting wear processes. In partial slip regime, the damage of the two coatings was very slight, and the porous structure was still intact even after cycles. The fretting wear mechanisms of the two coatings in slip regime were main abrasive wear and delamination, but higher proportion of the traces of relative sliding was presented on the scars of the sealed PEO coating.
As a conclusion, the sealed PEO coating exhibited a better resistance for alleviating fretting wear and lengthening service life than that of the PEO coating. The results showed that in unlubricated condition, the friction coefficient rapidly increased up to 0. In smear oil lubricated condition, the friction coefficient showed an obvious higher value within 0. While in oil bath lubricated condition, the friction coefficient reduced significantly to a low and stable value of 0. This indicated that the coatings with oil lubrication lowered the shear and adhesive stresses between contact surfaces, and consequently alleviated the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.
Employing liquid lubricants may improve the tribological properties of the PEO coatings. While in rigid and severe working conditions, such as high vacuum, high temperature, chemical and radioactive environments, liquid lubricants often do not function [ 45 ]. Furthermore, liquid lubricants may contaminate the workpieces.
Therefore, some multi-step preparation methods combined with the PEO process are employed to fabricate PEO-based duplex coatings on the metallic substrates. These duplex coatings can sharply decrease the friction coefficient and improve the wear resistance.
Herein, some successful applications for the duplex coatings were introduced briefly. Because the PEO ceramic coating is formed on the metal surface via a series of localized electrical discharge events, there are many micropores left in the coating [ 17 ]. This provides the probability to deposit small sized solid lubricant particles into these micropores to form a binary coating [ 46 ].
Herein, a simple and effective method of vacuum impregnation was introduced. In this method, the PEO coating samples are immersed into water-based solid lubricant suspension, and put into a vacuum oven. When deposited for a set time, the samples are heated for a period of time in high temperature for solidification. The solid lubricant particles can impregnate into the micropores of the PEO ceramic coating under the vacuum. With the increasing of deposition time and heat treatment, a compact top film covering the PEO ceramic coating is formed for anti-friction purpose.
Zhijiang Wang et al.
Surface Engineering of Metals: Principles, Equipment, Technologies
Results showed that the PTFE powder particles with the size in the range of nm could deposit into the PEO ceramic coating and covered the rough and porous surface. As the cracks and micropores of the PEO coating were filled by solid lubricant, PTFE could form a lubricating film on the frication surface of the steel ball when the steel ball which worked as counter material slided against the coating. With increased sliding distances, the solid lubricant provided continuous supply due to the abundance of PTFE lying in the micropores of the PEO coating. As a result, the friction coefficient of the self-lubricating coating could remain at a constant with minimal weight loss during the long-term sliding.
The porous feature of the PEO coating opens a good way to introduce solid lubricant into micropores or depositing on the surface of coating. And the presence of pores affords an effective mechanical keying between solid lubricant topcoat and the PEO layer. Spraying is a simple, effective and low cost method as a post treatment to apply solid lubricant on PEO coating to form a self-lubricating duplex coating. The PEO ceramic coating serves as underlying loading layer and solid lubricant top layer plays the roles as friction reducing agent.
Wang et al. Results showed that the surface of PEO coating was characterized by micropores of different size and shape and covered by graphite lubricant exhibiting a special shape of plate. The duplex coating exhibited good antifriction property, registering friction coefficient of about 0. Martini et al. The PTFE topcoat deposited by spraying a solvent-based aerosol suspension proves to be beneficial in terms of both friction and wear resistance, particularly in an intermediate N load range.
The friction coefficient of duplex coating reduced from 0. Deposition of thick e. Employing PEO technique can deposit thick ceramic coatings which exhibit high hardness, superior wear resistance and excellent load-bearing capacity. However, these PEO coatings generally exhibit high friction coefficients which can limit the wear resistance and cause the wear damage of counterpart materials.
In recent years, successful applications of these methods were done by some researchers as follows. Samir H. Awad et al.
The tribological properties were evaluated by ring-on-ring tests at speeds of 0. Results showed that the duplex coatings possessed very high hardness and wear resistance, and their mechanical and tribological properties were better than those of single TiN coatings, single PEO coatings and the uncoated Al alloy substrate. The Al2O3 intermediate layer played a crucial role in providing the load support essential to withstanding sliding wear at high contact loads.
Nie et al. Both the untreated Al alloy substrate and the PEO alumina coating gave a high friction coefficient of above 0. Whereas the single DLC coating on Al alloy failed quickly only at 25 m sliding distance due to its low load bearing capacity. For the uncoated Mg alloy substrate, the friction coefficient varied in the range of 0. Severe wear and seizing were observed.
Even for the DLC deposited Mg alloy substrate, the tribological behavior did not improve significantly. The DLC film failed quickly at around s after starting the sliding test, due to the low load-bearing capacity of soft substrates. The polished PEO coatings showed a very high friction coefficient of around 0. Arslan et al. For the PEO coatings, the friction coefficient was considerably high, approximately above 0.
For the single DLC coatings, the friction coefficient increased abruptly after seconds due to severe failure caused by their low load bearing capacity. The duplex coatings fabricated by the aforementioned methods can sharply decrease the friction coefficient and wear rate. However, these coatings are generally comprised of two layers: an inner PEO ceramic layer and an outside solid lubricant layer. When the outside layer is worn through, the tribological properties will be back to its original level.
Aliofkhazraei et al.
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