Nee Lam Loh

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Effects of pre-treatments and interlayers on the nucleation and growth of diamond coatings on titanium substrates

During diamond deposition on titanium substrates, two processes exist: (1) diffusion of hydrogen into a titanium substrate and the formation of hydride thereby degrading the mechanical properties of the substrate; and (2) competition among the rapid diffusion of carbon atoms into substrates, the formation of carbide and the nucleation of diamond crystals (thereby affecting the nucleation and growth rate of the diamond coating). To increase the diamond nucleation rate and prevent the rapid diffusion of hydrogen and carbon into the substrate, different surface treatments and interlayers were studied in this paper. Results showed that polishing with diamond pastes and ultrasonic pre-treatment in diamond suspensions will significantly increase the nuclei density of diamond crystals. However, the diffusion of hydrogen into the substrate could not be prevented. Pre-etching of the titanium substrate using hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Results showed that on a TiN interlayer, there was no significant improvement in diamond nucleation and growth, and the deposited diamond coatings showed poor adhesion. New diamond crystals were formed on the DLC interlayer in which DLC acted as the precursor for diamond nucleation. However, the so-formed diamond coating showed spallation. The plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, but results showed a relatively low nucleation density of diamond crystals and poor adhesion. A graded interlayer combining plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and carbon into the substrate and improving the nucleation rate and adhesion of diamond coating.

Structure and tribological properties of plasma nitrided surface films on Inconel 718

Abstract Inconel 718 specimen was plasma nitrided for microscopic examination and tribological testing. Film structure and composition, depth of film hardness and surface roughness were measured as a function of coating parameters. The film growth rate appears to be diffusion controlled, (i.e. case depth ∝ √treatment time). Nitrogen was detected in the surface of the nitrided layer by Auger electron spectroscopy. The nitrogen was found to react with Cr to form CrN which was detected in the nitrided layer by X-ray diffraction analysis. Friction and wear coefficients as a function of atmosphere, load, sliding speed, and surface temperature were performed. Exterior film structure varied from columnar or pyramidal with high roughness to a smoother spherical structure after nitriding at a lower temperature. The distribution of nitrogen within the film appeared uniform and there was a sharp boundary between the film and substrate. A maximum microhardness of 1290 HK was reached after prolonged nitriding. Nitrided surfaces showed 4 times lower coefficient of friction than a plain Inconel surface until the nitrided layer was worn away. Coarse lamellar wear particles showing signs of extreme plastic deformation were formed during wear of nitrided inconel.

Deposition of diamond coating on pure titanium using micro-wave plasma assisted chemical vapor deposition

The nucleation and growth of diamond coatings on pure Ti substrate were investigated using microwave plasma assisted chemical vapor deposition (MW-PACVD) method. The effects of hydrogen plasma, plasma power, gas pressure and gas ratio of CH4 and H2 on the microstructure and mechanical properties of the deposited diamond coatings were evaluated. Results indicated that the nucleation and growth of diamond crystals on Ti substrate could be separated into different stages: (1) surface etching by hydrogen plasma and the formation of hydride; (2) competition between the formation of carbide, diffusion of carbon atoms and diamond nucleation; (3) growth of diamond crystals and coatings on TiC layer. During the deposition of diamond coatings, hydrogen diffused into Ti substrate forming titanium hydride and led to a profound microstructure change and a severe loss in impact strength. Results also showed that pre-etching of titanium substrate with hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Plasma power had a significant effect on the surface morphology and the mechanical properties of the deposited diamond coatings. The effects of gas pressure and gas ratio of CH4 and H2 on the nucleation, growth and properties of diamond coatings were also studied. A higher ratio of CH4 during deposition increased the nuclei density of diamond crystals but resulted in a poor and cauliflower coating morphology. A lower ratio of CH4 in the gas mixture produced a high quality diamond crystals, however, the nuclei density and the growth rate decreased dramatically.

Friction and wear behaviour of carbon nitride films deposited on plasma nitrided Ti–6Al–4V

Amorphous carbon nitride (CNx) films were deposited on plasma nitrided Ti–6Al–4V substrate in order to improve the adhesion strength and tribological behaviour. Scratch and ball-on-disk wear tests were performed to evaluate the load bearing capacity, wear and friction characteristics of the duplex-treated coatings. Compared with a CNx film deposited on Ti–6Al–4V substrate, the load bearing capacity of a CNx film deposited on plasma nitrided layer was improved dramatically. Results showed that under dry sliding condition, the duplex-treated system was more effective in maintaining a favourable low and stable coefficient of friction and improving the wear resistance than both individual plasma nitriding and CNx film on Ti–6Al–4V substrate. The reasons for this significant improvement in tribological behaviour with the application of duplex treatment can be attributed to the combined benefits from both plasma nitriding and CNx films. (1) Plasma nitriding of Ti–6Al–4V produces a graded hardened case which serves as an excellent supporting and load bearing layer for hard CNx films. (2) The CNx film deposited at low temperature can produce a wear resistant and low-friction surface without impairing the beneficial effects from plasma nitriding treatment, and the smooth CNx films could effectively reduce both the interfacial stresses and the stresses near the surface thus providing a good tribological behaviour. (3) The graphitisation of the wear debris during dry sliding condition can help to decrease the coefficient of friction and improve the wear resistance.

Characterization and tribological evaluation of MW-PACVD diamond coatings deposited on pure titanium

Titanium alloys are widely used in aerospace and biomedical conditions, however, they are notorious for the poor tribological properties. The deposition of a well adherent diamond coating is a promising way to solve this problem. In this study, diamond coatings were deposited on pure titanium using microwave plasma assisted chemical vapour deposition (MW-PACVD). Characterisation of diamond coatings was performed using scanning electron microscopy (SEM), laser profilometry, Raman spectroscopy, grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Tribological properties of diamond coatings were evaluated using a ball-on-disk wear tester (sliding with Al2O3 balls) and a scratch tester (sliding with diamond pin). Results showed that the friction and wear properties of polycrystalline diamond coatings as well as the wear of the counterface were dependent significantly on the surface roughness, the morphology and crystalline structure of diamond coatings as well as the counterface materials. For (111)-textured diamond coatings with rough surface and sharp asperities sliding with Al2O3 balls, the coefficient of friction was much higher than that of (100)-textured coatings, and the wear of the counterface material was quite high. After polishing the diamond coating, the surface roughness, coefficient of friction and wear of counterface decreased significantly. If sliding with diamond pins, the coefficient of friction of diamond coating shows a quite low and stable value. To improve the tribological properties, a three-step deposition method was proposed to obtain a smooth and nano-crystalline diamond layer on bulk diamond coatings. The so-formed diamond coating showed the highest load bearing capacity, the lowest coefficient of friction and the lowest wear of the counterface.

Tribology of UHMWPE tested against a stainless steel counterface in unidirectional sliding in presence of model synovial fluids : part 1

Friction and wear tests have been performed on ultra-high molecular weight polyethylene (UHMWPE) in presence of proteins, dry sliding conditions against a steel counterface disc and the results have been analyzed in detail. UHMWPE was found to exhibit lowest friction coefficient and wear rates when lubricated with bovine α globulin and with bovine albumin respectively. Post-test analysis of the proteins indicated denaturing, formation of reaction products of specimens and proteins. The predominant wear mechanisms found were adhesion, abrasion and fatigue.

The wear properties of plasma transferred arc cladded stellite specimens

Abstract An investigation was carried out to study the wear properties of plasma transferred arc cladded stellite specimens tested at room and elevated temperatures, using the pin-on-disc testing technique and with a square pin of 9.9 mm and a length of 11 mm. Preliminary results indicated that the wear resistance of the specimens is dependent on the applied pressure (load), the disc rotating speed, disc material and the testing temperature. The wear rate, as measured by the weight loss of the pin, increases with increased applied load, reduced rotating speed, harder disc material or increased temperature. There are near linear relationships when the values of weight loss are plotted against the corresponding values of sliding distance. However, it is difficult to detect the transition point between the initial running-in (transient) and the equilibrium wear rates of the specimens using such a technique. It is found that the wear coefficient is a better parameter. The wear coefficient vs. sliding distance diagrams show more clearly the trend and changing wear characteristics of the specimens. The transition from the initial running-in to equilibrium wear rates can also be observed more easily.

Preparation and fretting wear behavior of ion-beam-enhanced-deposition CrN films

Abstract Hard chromium nitride films were deposited by ion-beam-enhanced-deposition (IBED) technique. The effects of ion beam energy and ion beam current on microstructure and mechanical properties of IBED CrN films were studied, and the optimized parameters were found to prepare the films to improve the fretting wear resistance of Ti–6Al–4V. The fretting wear behavior of the IBED CrN film was evaluated using a ball-on-flat system under unlubricated conditions, and the results were compared with those of PVD CrN films. Results showed that both the bombardment energy and ion beam current had significant effects on the microstructure, preferred orientation, hardness and adhesion strength of the IBED CrN thin films. The Fretting wear resistance of IBED CrN films was better than those of PVD CrN films due to a dense and fine microstructure, good adhesion strength, deep hardening depth, etc. Fretting parameters (normal load and amplitude) played an important role in the transition of the fretting regime (from partial slip to gross slip regime). Oxidation and abrasive wear were the main fretting wear mechanisms for IBED CrN film.

Effect of counterface on the tribology of UHMWPE in the presence of proteins

Artificial joints in orthopedics occupy a principal position owing to the increase in number of cases suffering from arthritis and associated diseases in addition to impairment caused by accidents. In this work, one of the most commonly used joint material, i.e. ultrahigh molecular weight polyethylene (UHMWPE), was tested against the duplex stainless steels instead of the conventional 316 L stainless steel. The UHMWPE was found to exhibit the lowest friction coefficient and wear rates when lubricated with water followed by globulin and glucose. The friction coefficient in the presence of egg albumen was higher along with high wear rates recorded. Post-test evaluation of surface roughness and wear scar/track analysis was performed to identify the wear mechanisms. Worn surfaces were analyzed using a differential scanning calorimeter for changes in crystallinity with sliding. The specimens tested under lubricated conditions with glucose, egg albumen and globulin indicated the presence of reaction products on the worn surface. Adhesive and corrosive wear mechanisms were the predominant modes of wear identified on the polymer samples. The wear tracks indicated that the proteins did react with the counterface material forming a thin deposit on them. Low temperature nitriding of the duplex stainless steel counterfaces were performed and the UHMWPE specimens were tested under similar conditions against the nitrided surfaces. Low temperature nitriding of the counterface did result in improved tribological behavior of UHMWPE and the corrosive effects were minimal.