Effect of laser surface melting on friction and wear behavior of AM50 magnesium alloy

In the present study, an attempt has been made to enhance tribological properties of AM50 magnesium alloy by laser surface melting (LSM) with a 2 kW continuous wave CO₂ laser. The microstructure of the laser surface melted zone consists of fine columnar dendrites growing epitaxially from the liquid-solid interface. Microhardness of the melted zone was improved to 55-75 HV as compared to 40 HV of the substrate. The friction and wear behavior of the laser surface melted layer were investigated using a ball-on-flat apparatus under dry sliding condition. It was found that the friction coefficient curve of the laser surface melted layer was similar to that of substrate. They showed a lower initial friction coefficient about 0.18 that after the running-in period increased up to about 0.38. Furthermore, compared with the AM50 substrate, the wear volume of the laser surface melted layer was decreased by 42%, the wear resistance of the laser surface melted layer was improved.     

Tribological properties of duplex treated TiN/TiCN coatings on plasma nitrided PH15-5 steel

Nitriding was carried out in low pressure plasma excited by single- or dual-frequency discharge modes, at a substrate temperature of 523 K, followed by the deposition of 3 μm thick TiCN or TiN/TiCN coatings at a PH15-5 substrate temperature of 723 K. The nitrided layer was comprised of two distinct sublayers, namely a compound layer and a diffusion layer, with a total thickness of ∼ 60 μm. The compound layer was γ′-Fe₄N and the diffusion layer was a solid solution of nitrogen in iron. The thickness of the compound layer fabricated by a single mode plasma is ∼ 5 μm, while that fabricated by dual-frequency mode plasma is ∼ 35 μm.It was found, using a ball-on-disk test, that the plasma nitrided layer fabricated by dual-frequency mode improved wear resistance by nearly one order of magnitude and improved the erosion resistance by a factor of two, compared with untreated steel. This improvement was common to the two nitriding treatments and both types of hard coatings. In particular, a thicker compound layer did not impair the wear resistance or the erosion resistance of the duplex treatment. The erosion resistance shows a linear dependence on the hardness of the uppermost nitrided or deposited layer.     

Effect of fretting amplitude and frequency on the fretting corrosion behaviour of tin plated contacts

The fretting corrosion behaviour of tin plated copper alloy contacts at 3, 10 and 20 Hz and at two different track lengths (fretting amplitude) of ± 5 and ± 25 μm is studied. The change in contact resistance as a function of fretting cycles, surface profile of the contact zone, extent of fretting damage, extent of oxidation and elemental distribution across the contact zone were used to assess the fretting corrosion behaviour. The time to reach a threshold value of contact resistance of 0.1 Ω is found to be early for the track length of ± 5 μm compared to that of ± 25 μm, at all the three frequencies. For a given track length, this threshold value reaches early at 20 Hz. The roughness and the nature of surface profile suggest considerable amount of oxidation have occurred at the track length of ± 25 μm compared to that of ± 5 μm. The surface morphology of the fretted zone reveals severe damage of the contact zone for samples with a track length of ± 25 μm at all the three frequencies. A pictorial model is proposed to describe the evolution of change in area of the contact zone. Based on the length and width of the contact zone, the fretted area is calculated. The change is fretted area as a function fretting frequency and track length is analyzed. Delamination wear is found to be operative at both track lengths and at all three frequencies. EDX line scanning also indicates higher levels of oxidation at the track length of ± 25 μm compared to that of ± 5 μm. The variation in the atomic ratios of tin, copper and oxygen of the oxide debris present at the centre and edges of the fretted zone is plotted as an area plot as a function of experimental conditions. The debris is predominantly oxides of copper for the track length of ± 25 μm whereas they are mostly oxides of tin for the track length of ± 5 μm at all the three frequencies. The narrow and deep surface profile, lower R_a values, overlapping of the tin and copper lines in the EDX line scan and the predominance of oxides of tin support the view that the chances of accumulation of wear debris at the contact zone is very high at the track length of ± 5 μm. The study concludes that tin plated contacts could encounter an early failure even at shorter track lengths of ± 5 μm, if there is sufficient accumulation of the wear debris at the contact zone.     

Studies on the development of TZM alloy by aluminothermic coreduction process and formation of protective coating over the alloy by plasma spray technique

TZM alloy is a potential candidate for high temperature structural applications. However, in the preparation of this alloy by conventional melt-casting route, difficulties are encountered in achieving homogenized alloy composition in view of high melting temperature of the alloy and presence of minor alloying components. Therefore, an alternative technique of aluminothermic co-reduction was adopted to prepare TZM alloy of composition, Mo-0.5Ti-0.1Zr-0.02 °C, wt.% by simultaneous reduction of uniformly premixed oxides of MoO₂, TiO₂ and ZrO2 by aluminium in presence of requisite amount of carbon. The as-reduced alloy was further arc melted for consolidation. Since, TZM alloy is by nature highly susceptible to oxidation at elevated temperature in air or oxygen, therefore feasibility of development of silicide type of coating over the synthesized alloy by plasma coating technique was also examined. Silicon powder coated on TZM alloy surface by plasma spray technique was finally converted into MoSi₂ coating by sintering at 1350 ^°C for 2-4 h duration under argon. A double layer coating structure was formed with two distinct phases. The inner thin layer was consisted of Mo₂Si₅ phase (~ 10 μm) followed by thick outer layer of MoSi₂ (~ 150 μm). The coating showed good adhesion strength and stable oxidation with negligible mass gain (10 g/m²) at 1000 ^°C in air.     

Microstructure and Dry Sliding Wear Behavior of Mg-Y-Zn Alloy Modified by Laser Surface Melting

Mg-11Y-2.5Zn alloy was surface-melted using a 6.0 kW continuous wave CO₂ laser as a heat-generating source. X-ray diffractometer, laser optical microscopy, and Vickers hardness indentation were used to characterize the microstructure and hardness of the Mg-11Y-2.5Zn alloy. The results show that the microstructure in the laser-melted zone can be greatly refined and hardness is slightly improved. Dry sliding tests were performed on the as cast and laser surface-melted Mg-11Y-2.5Zn alloys using a pin-on-disk configuration. Coefficients of friction and wear rates were measured within a load range of 20-320 N at a sliding velocity of 0.785 m/s. Laser surface-melted Mg-11Y-2.5Zn alloy exhibited good wear resistance when compared with the as cast one under given applied load conditions, which has been explained by refining of the microstructure in the melted zone. Morphologies of worn surface on the as cast and laser surface-melted Mg-11Y-2.5Zn alloys were examined using scanning electron microscopy. Four wear mechanisms, namely abrasion, delamination, thermal softening, and melting, have operated.     

The formation of a tungsten containing surface layer in a carbon steel by compression plasma flow

Morphology, phase and elemental composition of a carbon steel surface layer after treatment by compression plasma flows containing a dispersed tungsten powder have been investigated in this work. The action of relatively short (∼ 120 μs) and intense (15-20 J/cm² per pulse) plasma pulses resulted in the formation of tungsten containing thin film consisting of clusters with the size of 100-200 nm. Besides, the film formation treatment with a few pulses allowed to alloy a surface steel layer with tungsten. This treatment also led to the formation of Fe₃W₃C and WC carbides in the surface layer.     

Workpiece surface integrity when slot milling γ-TiAl intermetallic alloy

Slot milling is presented as a potential manufacturing route for aerospace component feature production when machining γ-TiAl intermetallic alloy Ti–45Al–2Mn–2Nb + 0.8 vol.% TiB₂XD using 2 mm diameter AlTiN coated WC ball nose end milling cutters. When operating with flood cutting fluid at v = 88 m/min, f = 0.05 mm/tooth, d = 0.2 mm, maximum flank wear was ∼65 μm after 25 min. SEM micrographs of slot surfaces show re-deposited/adhered and smeared workpiece material to a length of ∼50 μm. Brittle fracture of the slot edges was restricted to <10 μm with sporadic top burr formation observed up to ∼20 μm. Cross sectional micrographs of the slot sidewalls showed bending of the lamellae limited to within 5 μm.     

Laser surface cladding of MRI 153M magnesium alloy with (Al+Al2O3)

Laser surface cladding was carried out on a creep-resistant MRI 153M magnesium alloy with a mixture of Al and Al₂O₃ powders using a pulsed Nd:YAG laser at scan speeds of 21, 42, 63 and 84 mm/s. The Al₂O₃ particles partially or completely melted during laser irradiation and re-solidified with irregular shapes in the size range of 5-60 µm along with a few islands as large as 500 µm, within the grain-refined Mg-rich dendritic matrix. More than an order of magnitude improvement in wear resistance after cladding was attributed to the presence of ultra-hard Al₂O₃ particles, increased solid solubility of Al and other alloying elements, and a very fine dendritic microstructure as a result of rapid solidification in the cladded layer. However, corrosion resistance of the laser cladded alloy was reduced by almost an order of magnitude compared to that of the as-cast alloy mainly due to the presence of cracks and pores in the cladded layer.     

40Cr钢激光表面加Ni60B合金化及其磨蚀性能研究

     对40Cr钢进行了表面加Ni60B粉末激光合金化处理.金相、扫描电镜、X射线衍射分析,硬度测试和磨损与盐雾腐蚀实验的结果表明：合金化层的结构为熔化区、过渡区及热影响区;熔化区显微组织为胞状一树枝状晶,热影响区为极细的隐晶马氏体;激光合金化处理后的试样产生了新相Cr₂₃C₆和Cr₃C2,显微硬度Hk可达到8.6 GPa,比基体提高了近3倍;耐磨性与耐蚀性都比基体有明显提高.       

Broad-beam laser cladding of Al-Si alloy coating on AZ91HP magnesium alloy

In the present study, an attempt was made to improve the wear resistance and the corrosion resistance of AZ91HP magnesium alloy by laser cladding Al-Si eutectic alloy. The results showed that the clad layer mainly consisted of Mg₂Si, Mg₁₇Al₁₂ and Mg₂Al₃ phases. The microstructure of the bonding zone changed from columnar grains to equiaxial grains along the direction of heat-flow. The heat-affected zone consisted of α-Mg and α-Mg + β-Mg₁₇Al₁₂ eutectic. The formation of multiple Mg intermetallic compounds allowed the clad layer to exhibit higher hardness, better wear resistance and corrosion resistance.     

Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

LY12铝合金电子束表面改性层组织结构及腐蚀性能

对LY12铝合金表面进行高能电子束轰击改性处理,结果表明,合金表面出现熔坑,随轰击次数增加,熔坑数量减少,但直径变大。XRD分析得出,处理后,改性层中存在α-Al和第二相CuMgAl2,CuAl2,重熔层再结晶有择优取向。电化学腐蚀实验显示,经过轰击处理后,自腐蚀电位可从-626.92mV提高到-523.33mV,自腐蚀电流由1.09×10-7 A降低至3.12×10-9 A,耐腐蚀性能有明显提高。     

Structural and optical properties of diamond like thin films deposited by asymmetric bipolar pulsed-DC reactive magnetron sputtering

In contrast with PECVD technology, reactive sputtering of graphite allows an independent control of the substrate bias. This characteristic permits the modification of film properties without varying the plasma composition. In the present study, the characteristics of DLC films grown by pulsed-DC reactive sputtering were determined as a function of substrate bias. Asymmetric bipolar pulsed-DC in a gas mixture of Ar and 7.5% CH₄ with substrate bias in the range of − 300 V to 0 V, provided wear resistant a-C:H films with wear rate values in the range of 15 to 23 · 10^− 15 m³ m^− 1 N^− 1. DLC exhibit characteristics associated to hydrogenated DLC films (DLCH), namely: moderate sp³ content and mass density (up to 1.8 g/cm³), low hydrogen content (∼ 30%) and high transparency (> 90%) up to wavelengths of 700 nm with a Tauc gap energy up to 1.9 eV. Moreover, they also showed low stress values and moderate wear rates, as shown in previous studies.     

Tribological behavior of hafnium diboride thin films

Transition metal diborides and their coatings offer an excellent combination of high hardness, high chemical stability and high thermal conductivity, thus they are excellent candidates for a wide range of tribological applications. In this work, stoichiometric hafnium diboride films were grown by chemical vapor deposition from a single-source, heteroatom-free precursor Hf(BH₄)₄ under conditions that afford highly conformal and smooth films. HfB₂ films of thickness ∼ 0.6 μm deposited on steel substrates were subjected to pin-on-disk wear testing against a counter face disc of AISI 440C martensitic stainless steel. Based on wear measurements, both as-deposited (X-ray amorphous) and annealed (nanocrystalline) samples showed very high wear resistance compared to uncoated samples. For the annealed samples, SEM analysis indicated the formation of a wear resistant composite body in the wear scar, even at depths far exceeding the film thickness, which appears to dramatically improve wear resistance. No mild-to-severe wear transition was observed which indicates that mild wear occurred throughout the wear test. This ensemble of results, when considered in the light of high contact pressures (∼ 700 MPa) used in the study, makes the HfB₂ material potentially attractive for wear-resistant applications.     

Thermal stability of superhard Ti-B-N coatings

Ternary transition-metal boron nitride Ti-B-N offers outstanding hardness and thermal stability, which are increasingly required for wear resistant applications, as the protective coatings are subjected to high temperature, causing thermal fatigue. Ti-B-N coatings with chemical compositions close to the quasibinary TiN-TiB₂ tie line and boron contents below ∼ 18 at.% contain a crystalline supersaturated NaCl structure phase, where B substitutes for N. Annealing above the deposition temperature causes precipitation of TiB₂, which influence dislocation mobility and hence the hardness of TiB_0.40N_0.83 remains at a very high level of ∼ 43 GPa with annealing temperature T_a up to 900 °C. Growth of Ti-B-N coatings with B contents above ∼ 18 at.% results in the formation of nm sized TiN and TiB₂ crystallites embedded in a high volume fraction of disordered boundary layer. The compaction of this disordered phase during annealing results in a hardness increase of TiB_0.80N_0.83 coatings from the as-deposited value of ∼ 37 GPa to ∼ 42 GPa at T_a = 800 °C. Excess B during growth of TiB_2.4 coatings causes the formation of bundles of ∼ 5 nm wide TiB₂ subcolumns encapsulated in a B-rich tissue phase. This nanocolumnar structure is thermally stable up to temperatures of ∼ 900 °C, and consequently the hardness remains at the very high level of ~ 48 GPa, as nucleation and growth of dislocations is inhibited by the nm sized columns. Furthermore, the high cohesive strength of the B-rich tissue phase prevents grain boundary sliding.     

In situ synthesis of nanocrystalline intermetallic layer during surface plastic deformation of zirconium

By means of a surface plastic deformation method a nanocrystalline (NC) intermetallic compound was in situ synthesized on the surface layer of bulk zirconium (Zr). Hardened steel shots (composition: 1.0C, 1.5Cr, base Fe in wt.%) were used to conduct repetitive and multidirectional peening on the surface layer of Zr. The microstructure evolution of the surface layer was investigated by X-ray diffraction and scanning and transmission electron microscopy observations. The NC intermetallic layer of about 25 μm thick was observed and confirmed by concentration profiles of Zr, Fe and Cr, and was found to consist of the Fe_100 − xCr_x compound with an average grain size of 22 nm. The NC surface layer exhibited an extremely high average hardness of 10.2 GPa. The Zr base immediately next to the compound/Zr interface has a grain size of ∼ 250 nm, and a hardness of ∼ 3.4 GPa. The Fe_100 − xCr_x layer was found to securely adhere to the Zr base.     

Tribological behaviour and residual stress of electrodeposited Ni/Cu multilayer films on stainless steel substrate

In the present study, [Ni (4.5 nm)/Cu (t_Cu = 2, 4 and 8 nm)] multilayers were pulse electrodeposited on stainless steel (AISI SS 304) substrate from sulphate based single bath technique. X-ray diffraction (XRD) was used to investigate the structure and stress of the Ni/Cu multilayer. The results from XRD analysis indicated that the deposited multilayers had a preferred crystal orientation of [111] and presence of satellite reflection suggested the formation of superlattice. The stress level within the deposited multilayers was found to be sensitive to the sublayer thickness. Sliding wear behaviour of electrodeposited Ni/Cu multilayer films has been investigated against a tungsten carbide (WC) ball as the counter body and compared with that of the constituents, Cu and Ni coatings. The wear tests were carried out by using a reciprocating ball-on-flat geometry at translation frequencies of 5 and 10 Hz, slip amplitude of 1 mm and at five different loads of 3, 5, 7, 9 and 11 N. Friction force was recorded on-line during the tests. At the end of the tests, the wear scars were examined by laser surface profilometry and scanning electron microscopy (SEM). Friction coefficient was found to be dependent on load and Cu layer thickness (t_Cu) and the values for multilayers were border between Ni and Cu. Among multilayers, sample with minimum t_Cu has shown the lowest friction coefficient and wear rate. With increasing t_Cu, the wear mechanism changes from pure abrasive wear at t_Cu = 2 nm, to particle entrapment at t_Cu = 4 nm to particle embedding at t_Cu = 8 nm. Detailed investigation of the wear scar morphology as well as wear rate measurement revealed that at low loads, (H/E) ratio and residual stress governed the wear rate and the principle wear mode was abrasive cutting. At intermediate loads, the role of residual stress became insignificant while wear was governed by (H/E) ratio and plastic deformation. However, at higher loads, plastic deformation played the major role.     

Spatial stress distribution and optical properties of GaN films grown on convex shape-patterned sapphire substrate by metalorganic chemical vapor deposition

μ-Raman and μ-photoluminescence methods have been employed to investigate microscopic spatial stress distribution and optical properties of GaN films grown on the convex shape-patterned sapphire substrate (CSPSS). By comparison of the μ-Raman and μ-PL spectra, we found that significantly large difference, Δσ_xx ∼0.46 GPa, in biaxial compressive stress between the flat trench and convex regions in the side facet of the GaN film, around ∼2 μm below the surface whereas on the GaN surface, little difference with large residual stress was observed in both regions compared to those from the side facet. Temperature dependent and time-resolved photoluminescence spectra have shown that the GaN film grown on the CSPSS has improved crystal purity through the reduction of intrinsic point defects.     

Microstructure and mechanical performance of new Al0.5CrFe1.5MnNi0.5 high-entropy alloys improved by plasma nitriding

Effects of plasma nitriding at 525 °C on microstructure and mechanical performance of a brand-new Al_0.5CrFe_1.5MnNi_0.5 high-entropy alloy (HEA) were investigated. This alloy exhibits a large age hardening effect at temperatures from 600 to 800 °C and can be well-nitrided in the as-cast condition or the homogenized and furnace-cooled state. The nitrided layer has a thickness around 75 μm and a peak hardness level of Hv 1250 near the surface. The nitrided Al_0.5CrFe_1.5MnNi_0.5 alloys exhibit superior adhesive wear resistance to conventional nitrided steels by 25-54 times due to their much thicker highly-hardened layer and higher peak hardness than that of conventional steels.     

The effect of hot-dipped aluminum coatings on Fe-8Al-30Mn-0.8C alloy

The morphology and microstructure of an intermetallic layer formed on the surface of Fe-8Al-30Mn-0.8C alloy by hot-dip aluminization treatment have been examined in detail. The phases present in the coating are unambiguously identified by means of transmission electron microscopy. After aluminization, a two layer coating was formed consisting of an external Al layer and a (Fe, Mn)₂Al₅ intermetallic on top of the substrate. The (Fe,Mn)₂Al₅ compound has an orthorhombic structure with lattice parameters a = 0.752 nm, b = 0.667 nm and c = 0.417 nm. The activation energy (E_FeMnAl) for the growth of such an intermetallic layer is calculated to be 52.7 kJ/mol. These results are different from those observed in aluminized low-carbon steel (E_Fe). The difference between E_FeMnAl and E_Fe is attributed to the alloying elements (Mn) in the present alloy. Environmental salt fog corrosion and high temperature oxidation tests were carried out to examine the corrosion and oxidation resistance. The results indicated that both the corrosion and oxidation resistance of the Fe-8Al-30Mn-0.8C alloy treated by hot-dip aluminization can be significantly increased.