Optimization of diamond coated microdrills in aluminum alloy 7075 machining: A case study

Aluminum alloy 7075 is widely used for producing micro-scale heat sinks, micro-fluidic devices, micro-propellers and so on. This paper deals with optimizing microstructure and thickness of diamond coatings on microdrills used in 7075 aluminum alloy machining. Firstly, the friction tests between microcrystalline diamond (MCD), nanocrystalline diamond (NCD) films and aluminum alloy reveal that the stable coefficient of friction (COF) of MCD–aluminum alloy working pair is 0.240, much higher than that of NCD–aluminum alloy working pair (0.072). The decrease of COF is mainly attributed to the lower roughness of NCD films and the presence of more graphite or the non-diamond phases in NCD coatings. Afterwards, comparative cutting tests involving MCD, NCD, diamond-like coating (DLC) and TiAlN coated microdrills show that after drilling 200 holes, NCD coated microdrills exhibit the best cutting performance. Furthermore, NCD coated microdrills with coating thicknesses of 1 μm, 2 μm, 4.5 μm and 7 μm are fabricated and their cutting performance is studied in aluminum alloy machining. The cutting experiments show that the NCD coated microdrill with coating thickness of 4.5 μm shows the best cutting performance, exhibiting not only lowest flank wear and no tool tipping or chipping on the main cutting edges but also the highest quality of drilled holes because of the outstanding adhesive strength and wear resistance of the NCD coating.     

Hybrid diamond/graphite films: Morphological evolution,microstructure and tribological properties

Diamond films were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) using H₂ and CH₄ gas mixture. The morphological evolution process was characterized systematically by means of field-emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Special attention has been paid to the influence of the methane concentrations on the microstructures of diamond films, which shows a gradual transition from nanocrystalline to microcrystalline films, and finally displays a hybrid diamond-graphite nanostructure with the length of a few micrometers. Finally, the friction behaviour of the hybrid films was studied. The value of the friction coefficient of the hybrid films is 0.10 and the corresponding wear resistance is below 1.9 × 10^− 7 mm³/N·m in diamond composites/Al₂O₃ sliding system in ambient atmosphere under dry sliding conditions. It is a convenient path to synthesize hybrid diamond/graphite nanostructures by MPCVD depending on higher methane concentrations in the absence of nitrogen or argon. The structure is appropriate for the potential applications as high efficient mechanical tools.     

Frictional properties of diamond and fullerene nanoparticles sprayed by a high-velocity argon gas on stainless steel substrate

Lubricating abilities of diamond nanoparticles with size between 50 nm and 200 nm were studied in vacuum and in air to clarify the effective use of diamond fine powders for tribological purposes. Spraying of powders with a high-velocity argon gas jet was performed to form deposits on stainless steel (SUS304) substrates. For sliding in vacuum against SiC and Al₂O₃ balls under a 0.5 N applied load and 3.5 mm/s sliding speed, the deposits of microcrystalline diamond powders with a mean particle size of 50 nm and detonation nanodiamond with a mean aggregate size of around 75 nm demonstrated friction coefficients of less than 0.01 and 0.03, respectively, and ball wear rates of less than 2 · 10^− 6 mm³/(Nm). This means that diamond fine powders smaller than 100 nm can be considered as good solid lubricants in vacuum, because they demonstrate not only a low friction coefficient, but also wear rate of SiC ball lower than non-lubricated SUS304 does. A C₆₀ deposit, formed by the same method on the SUS304, was readily scratched from the substrate in vacuum; however, under open-air conditions, a friction coefficient of around 0.1 and a SiC ball wear rate of about 2 · 10^− 6 mm³/(Nm) were observed. This fact calls attention to the influence of the deposition method on C₆₀ frictional properties.     

Effect of temperature on friction and wear behaviour of CuO–zirconia composites

Results of wear tests using an alumina ball sliding against 5 wt% copper oxide doped tetragonal zirconia polycrystalline (CuO-TZP) ceramics are reported as a function of temperature up to 700 °C. The specific wear rate and friction coefficient are strongly dependent on temperature. Below a critical temperature (T < 600 °C), CuO-TZP showed a high coefficient of friction as well as a high wear rate. This was ascribed to the formation of a rough surface, caused by brittle fracture and abrasive wear, based on observations by scanning electron microscopy (SEM), laser scanning microscopy (LSM) and X-ray photoelectron spectroscopy (XPS). However, above 600 °C a self-healing layer is formed at the interface and results in low friction and wear. The mechanism of layer formation and restoration is discussed and rationalized by onset of plastic deformation caused by a reduction reaction of CuO to Cu₂O at high temperatures.     

Mechanical properties of different types of diamond

The mechanical properties of different types of diamond (synthetic diamonds with different nitrogen impurity concentrations 0.3 and 200 ppm) have been investigated by sclerometry hardness and wear resistance measurements. Diamond (111) and (100) faces in the 〈100〉 and 〈110〉 directions were tested. It was found the synthetic diamond with nitrogen impurity concentration of 0.3 ppm exceeds other diamond types with respect to hardness and wear resistance, and reveals anisotropy of the mechanical properties, different from other diamond types. The hardness measured on the (111) face for synthetic diamond was 175±5 GPa for 0.3 ppm of nitrogen impurities and 151±5 GPa for 200 ppm of nitrogen impurities. The hardness measurements were performed using an ultrahard fullerite indenter exceeding diamond in hardness and the diamond faces were deformed plastically under scratching conditions.     

Wear mechanisms of TiN–TiB2 ceramic in sliding against alumina from room temperature to 700 °C

TiN–TiB₂ ceramic was prepared by the reactive hot-pressing method using titanium and BN powders as raw materials. The friction and wear properties of TiN–TiB₂ ceramic were evaluated in sliding against alumina ball from room temperature to 700 °C in air. The TiN–TiB₂ ceramic has a relative density of 98.6%, a flexural strength of 731.9 MPa and a fracture toughness of 8.5 MPa m^1/2 at room temperature. The TiN–TiB₂ ceramic exhibits a distinct decrease in friction coefficient at 700 °C as contrasted with the friction data obtained at room temperature and 400 °C. Wear mechanisms of TiN–TiB₂ ceramic depend mainly upon testing temperature at identical applied loads. Lubricious oxidized products caused by thermal oxidation provide excellent lubrication effects and greatly reduce the friction coefficient of TiN–TiB₂ ceramic at 700 °C. However, abrasive wear and tribo-oxidation are the dominant wear mechanisms of TiN–TiB₂ ceramic at 400 °C. Mechanical polishing effect and removal of micro-fractured grains play important roles during room-temperature wear tests.     

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti₃SiC₂ and Ti₃SiC₂/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800 °C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25 °C (RT) to 600 °C (friction coefficient ≈0.61–0.72, wear rate ≈10⁻³ mm³ N m⁻¹). An amount of Pb in the composites played a key role in lubricating with the temperature below 800 °C. The friction coefficient (≈0.22) and wear rate (≈10⁻⁷ mm³ N m⁻¹) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti₃SiC₂/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O₂=2PbO (below 600 °C) and 6PbO+O₂=2Pb₃O₄ (800 °C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.     

Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si₃N₄. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 °C, 300 °C, 500 °C, and 700 °C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 °C and 500 °C) coefficient of friction of monolithic Si₃N₄ and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 °C differences between the experimental materials disappeared and severe wear regime dominated in all cases.     

Enhanced sealing performance with CVD nanocrystalline diamond films in self-mated mechanical seals

Nanocrystalline diamond (NCD) films were evaluated as protective tribo-coatings on silicon nitride mechanical seal rings. The NCD films were deposited by microwave plasma assisted chemical vapour deposition (MPCVD) method from a CH₄/H₂/N₂ gas mixture. The sealing performance and friction behaviour of self-mated NCD films were assessed using the ring-on-ring tribological test in planar configuration varying the rotating speed and the applied load. Water sealing conditions were obtained in the P · V (P, the effective pressure and V, the linear speed) range of 0.5–4.8 MPa ms^− 1. The high hardness and smoothness of the NCD films resulted in a very low and stable friction coefficient value of 0.01, without any measurable wear.     

Pressureless sintering and tribological properties of WC–ZrO2 composites

In a recent work [Basu, B., Lee, J. H. and Kim, D. Y., Development of WC-ZrO₂ nanocomposites by spark plasma sintering. J. Am. Ceram. Soc. 2004 87(2), 317–319], the processing of ultrahard WC–ZrO₂ nanocomposites using spark plasma sintering is reported. In the present work, we investigate the processing and properties of WC–6 wt.% ZrO₂ composites, densified by pressureless sintering route. The densification of the WC–ZrO₂ composites was performed in the temperature range of 1500–1700 °C with varying time (1–3 h) in vacuum. The experimental results indicate that significantly high hardness of 22–23 GPa and moderate fracture toughness of ∼5 MPa m^1/2 can be obtained with 2 mol% Y–stabilized ZrO₂ sinter-additive, sintered at 1600 °C for 3 h. Furthermore, the friction and wear behavior of optimized WC–ZrO₂ composite is investigated on a fretting mode I wear tester. The tribological results reveal that a moderate coefficient of friction in the range from 0.15 to 0.5 can be achieved with the optimised composite. A transition in friction and wear with load is noted. The dominant mechanisms of material removal are tribochemical wear and spalling of tribolayer.     

Influence of countersurface materials on dry sliding performance of CuO/Y-TZP composite at 600 °C

Dry sliding wear tests on 5 wt.% copper oxide doped yttria stabilized zirconia polycrystals (CuO–TZP) composite have been performed against alumina, zirconia and silicon nitride countersurfaces at 600 °C. The influences of load and countersurface materials on the tribological performance of this composite have been studied. The friction and wear test results indicate a low coefficient of friction and specific wear rate for alumina and zirconia countersurfaces at F = 1 N load (maximum Hertzian pressure ～0.5 GPa). Examination of the worn surfaces using scanning electron microscope/energy dispersive spectroscopy confirmed the presence of copper rich layer at the edge of wear scar on the alumina and zirconia countersurfaces. However, Si₃N₄ countersurface sliding against CuO–TZP shows a relatively higher coefficient of friction and higher wear at 1 N load condition. These results suggest that the countersurface material significantly affect the behavior of the third body and self-lubricating ability of the composite.     

Wear behaviour of hydrogen free diamond-like carbon thin films in diesel fuel at different temperatures

In the present study, super hard, hydrogen free amorphous diamond-like carbons with a high fraction of sp³ hybridised carbon were deposited by pulsed laser deposition. The tribological performance of DLC coatings was investigated by translatory oscillating relative motion of a 100Cr6 steel ball in diesel fuel or ambient air at 25 °C or 150 °C temperature. The structure of the coatings and the tribological worn surfaces were characterised by Raman spectroscopy and by scanning electron microscopy. Bio-fuel with a high fraction of unsaturated fatty acids has the potential to reduce friction in tribological systems with chemically inert DLC. Diesel blend with 10% bio-fuel reduces friction at 150 °C. If there is no diesel fuel, pre-oxidation at 450 °C for 8 h leads to the best wear resistance (↓ f & wear rate) at room temperature. Without diesel fuel, enhancement of temperature up to 150 °C during wear testing causes an increase of the coefficient of friction. Again the 450 °C pre-oxidised sample revealed the lowest friction. For this coating, Raman spectroscopy points to a small increase of the sp² CC bonds. Diesel fuel seems to promote coherent coating failure under 150 °C wear, while pre-oxidation at 450 °C support adhesive coating ablation under higher loads or cyclic loading.     

Hot filament CVD diamond coating of TiC sliders

Hot filament chemical vapour deposition (HF CVD) process with low filament temperature (∼ 1650 °C) was utilized for the diamond coating of TiC samples. Porous substrates were fabricated by pulse discharge sintering (PDS) to create more nucleation sites. Nucleation density and morphology of deposited diamond films were studied using scanning electron microscopy (SEM). It was found that the highest growth rate occurs at substrate temperature of 980 °C. Evaluation of the residual stress in deposited films was carried out by Raman spectroscopy. Ball on disk tests were performed with steel as a counterface material. After polishing diamond films demonstrated good sliding performance: friction coefficient of 0.08 and wear rate of 10^− 17 m³/N m.     

High-rate homoepitaxial growth of CVD single crystal diamond by dc arc plasma jet at blow-down (open cycle) mode

Growth and applications of large size high quality single crystal diamond is one of the most significant progresses in the field of CVD diamond film research ever made in the past 15 years. However, up to now most of the works were done by microwave plasma CVD operating at high pressures. In the present investigation it is demonstrated that relatively high quality single crystal diamond layer with the FWHM of the diamond Raman peak of less than 2 cm^− 1 and the FWHM of the diamond (400) reflection X-ray Rocking Curve of 0.028° can be obtained by the 20 kW dc arc plasma jet operating at blow down (open cycle) mode at a growth rate as high as 36 μm/h. The reason why dc arc plasma jet is also suitable for high quality epitaxial growth of single crystal diamond is that very high concentration of atomic hydrogen can be easily provided by the extremely high gas temperature due to the arc discharging. Detailed results on the effects of the ratio of H₂/Ar, the distance between the substrate to the anode nozzle of the arc plasma torch, the concentration of methane, and the substrate temperature on the growth of single crystal diamond are presented, and discussed comprehensively, and compared to that with the MWCVD, and with our previous work on the 30 kW dc arc plasma jet operating at arc root rotation and gas recycling mode.     

Friction and wear properties of scrap tire/potassium hexatitanate whisker composites

As a way of solving the environmental problem of waste tires, we developed a new type of friction material made of scrap tire composites with potassium hexatitanate in which rubber formed a continuous phase. The tribological behaviors of the scrap tire rubber composites were investigated by a friction and wear tester under dry conditions. According to the results, the optimum amounts of potassium hexatitanate were 5 phr in terms of the friction and wear properties up to 200 °C. The specimen containing other ingredients showed 0.72 of friction coefficient and 1.03 of wear rate which are highly compatible to those of the commercial ‘Sonata’ motor brake pad when it contains 5, 20, 10, 20, 10 phr of potassium hexatitanate, phenol, cashew, barium sulfate, and copper, respectively.     

Role of transfer layer on tribological properties of nanocrystalline diamond nanowire film sliding against alumina allotropes

The tribological properties of nanocrystalline diamond nanowire (DNW) film treated in CH₄ atmosphere at 400 °C were studied in ambient atmosphere at room temperature using various allotropes of alumina ball as sliding counterbodies. Super low value of friction coefficient (~ 0.003) and extremely high wear resistance (~ 2.8 × 10^− 21 mm³/Nm) were observed when the Al₂O₃ ball slides against the film. In contrast, high friction coefficients with the values ~ 0.06 and ~ 0.07 were observed while using sapphire and ruby balls, respectively. Wear loss was also high ~ 4 × 10^− 19 mm³/Nm and 2.8 × 10^− 15 mm³/Nm in DNW/sapphire and DNW/ruby sliding pairs, respectively. Such a behavior is fundamentally explained in terms of the chemical nature of the sliding interfaces and surface energy of ball counterbodies. As a consequence, the chemical affinity of Al₂O₃ ball towards the carbon atoms is less, which resulted in the absence of carbonaceous transfer layer formation on the Al₂O₃ ball scar. However, in the case of sapphire and ruby balls, the wear track was found to be highly deformed and significant development of carbonaceous transfer layer was observed on respective ball scars. This phenomenon involving transfer layer formation is related to high surface energy and strong chemical affinities of sapphire and ruby balls towards carbon atoms. In such a condition, sliding occurs between film and the carbonaceous transfer layer formed on the ball exhibiting high energy due to covalent carbon bonds that chemically interact and enhance sliding resistance.     

Understanding the ultra-low friction behavior of hydrogenated fullerene-like carbon films grown with different flow rates of hydrogen gas

Fullerene-like hydrogenated carbon (FL-C:H) films that exhibit ultra-low friction and wear in humid conditions have been the subject of extensive researches, but the structure–performance relationship such as the evolution of FL structures under friction is not well understood. We have prepared FL-C:H films with different FL content, and have addressed a detailed investigation on the relationship. It is found that with the increase in FL content, the friction and wear of FL-C:H films can reach as low as 0.011 and 1.48 × 10⁻⁸ mm³/Nm, respectively. Examination of the corresponding wear tracks by Raman spectroscopy reveals that not graphitization but friction-induced promotion of FL structures causes the ultra-low friction and wear of FL-C:H films. We therefore claim that FL structures are in close positive relations with the excellent tribological performance of FL-C:H films.     

Influence of temperature and concentration on tribological properties of silicon nitride in glycerol aqueous solution

Friction and wear behaviors of self-mated Si₃N₄ in glycerol aqueous were investigated by varying the temperature (30 °C, 50 °C, and 70 °C) and concentration (pure water, 5 vol%, 20 vol%, and 50 vol%) of glycerol aqueous solution. Friction tests were conducted on a ball-on-disk apparatus. Normal load and sliding velocity were fixed at 30 N and 0.5 m/s, separately. After each tests, friction coefficients and wear rates were measured to evaluate friction and wear behavior. The results showed that the period of running-in process reduces with the increase of concentration and decrease of temperature. Increase of temperature could intensify wear behavior, and when concentration is larger than 20 vol%, wear rate of glycerol aqueous solution is one order less than that of pure water. Our findings could also guide for the use of glycerol aqueous solution as lubricant at different temperature. At 30 °C, the higher the concentration was, the smaller wear volume and total wear rate were. However, at 50 °C and 70 °C, total wear rates of disk were the largest when concentration is 5 vol%, a concentration of glycerol larger than 20 vol% must be added into water to reduce the wear rate. Wear regimes at different conditions were also given in this paper based on lubrication state number.     

Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (R_a = 381 nm) when compared to the tubes drawn through a regular carbide die (R_a = 527 nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.     

Microstructure evolution and wear resistance of nitride/aluminide coatings on the surface of Ti-coated 2024 Al alloy during plasma nitriding

A duplex surface treatment consisting in depositing a Ti film followed by plasma nitriding was adopted to improve the wear resistance of 2024 Al alloys. Nano-grained Ti films were firstly deposited on the substrate surface by using magnetron sputtering, then plasma nitrided for 8 h at 400 °C, 430 °C, 460 °C and 490 °C, in a gas mixture of 40% N₂+60% H₂. Duplex coatings composed of three sublayers (i.e. the outmost TiN_0.3 layer, the intermediate Al₃Ti layer and the inside Al₁₈Ti₂Mg₃ layer) were obtained at nitriding temperature higher than 460 °C. The coatings obtained at 400 °C and 430 °C consisted of mainly α-TiN_0.3 with (002) preferred orientation. The surface hardness of the coatings increased at higher nitriding temperature, reaching the maximum of 500 HV at 490 °C, which was about 8 times higher than that of the uncoated alloy. The friction coefficients of 2024 Al alloy decreased in the coatings prepared at higher nitriding temperature, reaching the lowest values of 0.31 at 490 °C. The wear rate of the coated samples decreased by 56% compared with the uncoated ones. The analysis of worn surface indicated that the nitrided samples exhibited severe adhesive wear at 400 °C that changed to predominant abrasive wear at increased nitriding temperature.