Fretting wear behaviour of surface mechanical attrition treated alloy 718

Alloy 718 was subjected to surface mechanical attrition treatment (SMAT) using SAE 52100 steel balls of a 5 mm diameter for four different treatment durations (15, 30, 45 and 60 min). Fretting wear tests were conducted at different normal loads on untreated and treated samples for 25,000 cycles using alumina as a counterbody material. Microstructural features of the surface layer of samples treated by SMAT were characterized by cross-sectional optical microscopy and transmission electron microscopy. Hardness, surface roughness and residual stress were determined using a nano-indenter, surface roughness tester and X-ray residual stress analyzer respectively. SMAT resulted in the formation of nanocrystallites on the surface and near surface regions, increased hardness, increased surface roughness and compressive residual stress at the surface. Treated samples exhibited lower tangential force coefficient (TFC) compared to untreated samples. Samples treated for 60 min exhibited higher grain refinement, higher hardness, lower surface roughness and higher TFC compared to the samples treated for 30 min. The wear volume and wear rate of samples treated for 30 min were lower compared to those of the untreated samples, which may be attributed to an optimum combination of hardness and toughness and a low work hardening rate of the nanocrystalline structure at the surface of the treated samples. In contrast, the wear volume and wear rate of the samples treated for 60 min were higher than those of untreated samples, presumably due to the higher hardness and reduced toughness of the samples treated for 60 min.     

Surface characterization of Ti-7.5Mo alloy modified by biomimetic method

Titanium and its alloys have been used in dentistry due to their excellent corrosion resistance and biocompatibility. It was shown that even a pure titanium metal and its alloys spontaneously form a bone-like apatite layer on their surfaces within a living body. The purpose of this work was to evaluate the growth of calcium phosphates at the surface of the experimental alloy Ti-7.5Mo. We produced ingots from pure titanium and molybdenum using an arc-melting furnace. We then submitted these ingots to heat treatment at 1100 °C for one hour, cooled the samples in water, and cold-worked the cooled material by swaging and machining. We measured the media roughness (Ra) with a roughness meter (1.3 and 2.6 μm) and cut discs (13 mm in diameter and 4 mm in thickness) from each sample group. The samples were treated by biomimetic methods for 7 or 14 days to form an apatite coating on the surface. We then characterized the surfaces with an optical profilometer, a scanning electron microscope and contact angle measurements. The results of this study indicate that apatite can form on the surface of a Ti-7.5Mo alloy, and that a more complete apatite layer formed on the Ra = 2.6 μm material. This increased apatite formation resulted in a lower contact angle.     

Joining and microstructural development of Ni-Al-Ti sheets under ball collisions

Ni-Al-Ti sheets were fabricated using a ball-collision technique at room temperature in an ambient atmosphere. The 0.5 mm thick Ni, Al and Ti foils were stacked and fixed at the top of a vibration chamber and ball-treated for durations of between 5 and 30 min. The microstructural evolution of the sheets was investigated as a function of the processing time and the mass of the ball. Joining of materials was considered to have occurred when materials at the interface plasticized, flowed and came into intimate contact under the high pressure developed by the ball collisions. The key parameters of the process were impact energy and processing time. Larger balls with higher impact energy were more effective for joining, and smaller balls were more effective for grain refinement. The transformation difference in grain refinement was attributed to the impact frequency, which appeared to be higher for the smaller balls. Ball collisions may have refined the grains of the Ni and Ti sheets to the nanometer scale, thereby destroying the initial rolling texture and inducing the formation of the fiber texture. The formation and stability of the 〈1 1 1〉 fiber texture in Ni were affected by the ball mass.     

机械研磨工艺对 AZ91D 镁合金显微结构的影响

目的研究不同机械研磨处理条件对AZ91D镁合金表面晶粒细化行为与机制的影响。方法采用不同直径的弹丸对密排六方结构的AZ91D镁合金进行不同时间的表面机械研磨处理,对处理试样的表面显微结构进行对比和表征,分析晶粒细化机制。结果通过表面机械研磨处理可以实现AZ91D镁合金的表层晶粒细化。在一定范围内,弹丸直径越大,处理时间越长,表层晶粒细化就越明显,晶粒尺寸可细化至约24 nm。结论 AZ91D镁合金表面机械研磨处理晶粒细化是孪生和位错滑移的综合结果。选择弹丸直径16 mm,处理时间180 min的工艺参数,AZ91D镁合金表面机械研磨处理的效果最好。     

Fabrication of nanostructured Mo coatings on Al and Ti substrates by ball impact cladding

Nanostructured multicomponent Mo coatings were fabricated on Ti and Al substrates by ball impact cladding at room temperature in an ambient atmosphere. The process involved subjecting the substrate and Mo foil fixed at the top of a vibration chamber to high-energy collisions with balls. The coating formation was the result of a simultaneous process of mechanically induced plastic flow, nanocrystallization, and interdiffusion caused by the ball collisions. Plastic deformation refined the grains at the Mo foil/substrate interface to the nanometre scale. The size of nanocrystalline grains in the Mo coatings ranged between 2 nm and 10 nm. The ball collisions caused atomic level intermixing of different elements, introduced into the surface from the steel balls used for milling, and solid solubility improved remarkably. The hardness of the Mo coatings on the Al and Ti substrates was 552 and 1010 HV, respectively. The initial hardness of the Mo foil was 287 HV. The high hardness of the Mo claddings was attributed to the fine grain structure, formation of supersaturated solid solutions, and high residual compressive stresses.     

Changes in the microstructure and hydrogen storage properties of Ti-Cr-V alloys by ball milling and heat treatment

Changes in the microstructure and hydrogen storage properties of Ti-Cr-V alloys were investigated after a combination of ball milling and heat treatment. Two different sets of balls and vials made of tungsten carbide (WC) and stainless steel (STS) were used for milling the samples. Ball milling using WC balls and vials induced WC contamination, and it caused compositional changes in the matrix during heat treatment. When STS balls and vials were used, meanwhile, no peak of the second phase caused by contamination was found in the X-ray diffraction (XRD) data. In the case of the sample that completed only the milling process, the crystallite size calculated from the XRD data, 20-30 nm, agreed well with the grain size obtained from transmission electron microscopy (TEM). On the other hand, for the sample that was heat treated after milling, the strain decreased from 0.74% to 0.18%, the crystallite size increased to 70-80 nm, and the grain size grew up to the level of hundreds of nanometers. The changes in microstructure induced by the ball milling and heat treatment influenced the hydrogen storage properties, such as plateau pressure, hysteresis, and phase transformation with hydrogen absorption. Thus, the relationship between the microstructure and hydrogen storage properties can be explained.     

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.     

The effect of nitric acid surface treatment on CaP deposition of Ti6Al4V open-cell foams in SBF solution

The effect of nitric acid surface treatment on CaP deposition of an open-cell Ti6Al4V foam (60% porous and 300-500 ?m in pore size), prepared by means of the space holder method using 94 and 66 μm average particle size powders, was investigated in a simulated body fluid (SBF) solution up to 14 days. Although, nitric acid surface treatment did not change the foam flat surface roughness values significantly, it increased surface area difference greatly by introducing nano scale undulations on the surface. The increased surface area difference was found to be more pronounced in smaller particle size foam samples. A continuous relatively thin CaP coating layer formed after 5 and 14 days of SBF immersion in nitric acid surface treated small and larger average particle size foam specimens, respectively. Whereas, the cells of untreated foam specimen were observed to be filled with CaP precipitates and a continuous CaP layer development was found after 14 days of SBF immersion. These results were also confirmed with the grazing incidence XRD and FTIR analysis of SBF immersed specimens.     

变曲率沟槽精密球研磨加工优化实验研究

目的获得变曲率沟槽加工方法研磨精密轴承钢球的最优工艺参数。方法应用田口法对变曲率沟槽加工方法研磨球体的参数进行实验和优化,以研磨压力、磨料粒径、磨料浓度为主要影响参数设计正交实验,以材料去除率、表面粗糙度和球度误差为评价指标,通过平均响应分析和方差分析得到最优研磨参数组合。结果对于材料去除率,研磨压力的影响最显著,磨料粒径的影响次之,磨料浓度影响最小;对于表面粗糙度,磨料粒径的影响最大,磨料浓度的影响次之,研磨压力影响最小;对于球度误差,压力的影响最大,其他因素的影响较小。结论在每球的研磨压力为5 N、磨料粒径为3000~#(5μm)、磨料质量分数为25%的条件下,球体的材料去除率最大,可达到0.28 mg/h。在磨料粒径为5000~#(3μm)、磨料质量分数为25%、每球研磨压力为2.5 N的条件下,球体的表面质量最佳,表面粗糙度最小达到12 nm。在每球研磨压力为0.5 N、磨料粒径为3000~#(5μm)、磨料质量分数为50%的条件下,球度误差小。     

Synthesis process of Mg–Ti BCC alloys by means of ball milling

The synthesis process of Mg–Ti alloys with a BCC (body centered cubic) structure by means of ball milling was studied by X-ray diffraction and various microscopic techniques. The morphology and crystal structure of Mg–Ti alloys changed with increase of milling time. During ball milling of Mg and Ti powders in molar ratio of 1:1, firstly, plate-like particles stuck on the surface of the milling pot and balls. After these plate-like particles fell off from the surface of the milling pot and balls, spherical particles with the mean diameter of 1 mm, in which concentric layers of Mg and Ti were disposed, were formed. These spherical particles were crushed into spherical particles with the diameter of around 10 μm by introduction of cracks along the boundaries between Mg and Ti layers. Finally, the Mg₅₀Ti₅₀ BCC phase with the lattice parameter of a = 0.342(1) nm and the grain size of 3 nm was formed. During milling of Mg and Ti to synthesize the BCC alloy, Mg and Ti were deformed mainly by the basal plane slip and the twinning deformation, respectively. Ti acted as abrasives for Mg which had stuck on the surface of the milling pot and balls. The BCC phase was found after Mg dissolved in Ti.     

Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion

The effect of cooling rate and substrate gauge upon the microstructure and corrosion resistance of Galfan (Zn-4.5 wt.%Al) coated steels is presented. The coatings, applied to steel of gauges 0.47 mm (light gauge) and 0.67 mm (heavy gauge) on a coil coating line, were subjected to three different cooling rates by increasing output from 55% to 100% of the total power from a high powered cooling rig. The increase in cooling rate did not significantly alter the volume fraction of the primary zinc, this remaining at ∼20%. However, the size and number of the primary zinc dendrites were altered. The fast cooled samples contained small but numerous (∼3000 mm⁻² in the heavy gauge and ∼2850 mm⁻² in the light gauge) dendrites as opposed to the slow cooled samples where there were fewer (∼1850 mm⁻² for the heavy gauge and ∼1500 mm⁻² in the light gauge) dendrites of greater size. Characterisation of the surface revealed a reduction in eutectic cell size (∼1.8 mm to ∼0.8 mm on the heavy gauge and ∼2.1 mm to ∼1.2 mm on the light gauge) with increasing cooling rate. This leads to an increased unit length of depressed boundary between the eutectic cells. The eutectic microstructure is also finer (with reduced inter-lamella spacing) in the fast cooled samples again reflecting the more rapid nucleation of the coating.The scanning vibrating electrode technique (SVET) has been used to quantify the effects of these microstructural changes upon the surface and cut edge corrosion performance. There is an increase in corrosion activity on the surface of the fast cooled samples (metal loss 150 μg to 260 μg on the heavy gauge and 50 μg to 80 μg on the light gauge) primarily due to the increased length of depressed boundaries. Applying the same analysis to the cut edge, a decrease in corrosion occurs upon the faster cooled specimens. Metal loss calculations show a decrease (140 μg to 75 μg on the heavy gauge and 190 μg to 115 μg on the light gauge) as the cooling rate is increased. The higher intensity long lived anodes at the cut edge in the slower cooling rate samples are directly related to the increase in zinc dendrite size within the coating as nucleation rates are reduced.     

High temperature processing of poly-SiC substrates from the vapor phase for wafer-bonding

The transfer by wafer-bonding of single-crystalline SiC thin films to a polycrystalline SiC substrate to obtain a “quasi-wafer” requires high quality polycrystalline substrates with controlled bulk properties (thermal conductivity, electrical resistivity) as well as with very low surface roughness (RMS < 5 nm) and bowing (< 10 μm). Currently, available polycrystalline SiC wafers are processed by sintering or by Chemical Vapor Deposition (CVD). Sintered ceramic wafers are very heterogeneous (mixture of 3C, 6H, 4H and silicon), while CVD ones are of better quality (homogeneous and textured 3C). The aim of this paper is to investigate the fabrication and the properties (bulk and surface) of SiC substrates with large (0.1 to a few mm) grains. To meet these requirements, two high temperature processes (around 2000 °C) for single crystal growth were used: Physical Vapor Transport (PVT) and the recently developed CVD Feed Physical Vapor Transport (CF-PVT). Structural investigations performed on large grain wafers sliced and polished from the grown ingots showed an important influence of the initial seed on the grain size, polytype and crystallographic texture. Chemical and Mechanical Polishing (CMP) of such structures was studied and optimized to obtain low surface roughness. The intra-grain roughness is very low (RMS < 0.5 nm) but a few nanometer of height steps were observed between grains. The relations between bulk properties, surface functionalization and process conditions are discussed. This first seeding step with commercial substrates is necessary for the creation of original substrates which can be used for the fabrication of new substrates.     

Tribological properties of nanoporous anodic aluminum oxide film

Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28 nm, 45 nm, 95 nm, and 200 nm diameter and 60-100 μm thick) were synthesized by anodization. Reciprocating wear tests using 1 mm diameter steel balls as counterpart were carried out for a wide range of load (from 1 mN to 1 N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1 N and 1 N) whereas it was negligible at low loads (1 mN and 10 mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size.     

Influence of N ion implantation on the corrosion and nano-structure of Ti samples

Nitrogen ions of 30 keV with different fluxes ranging from 5 × 10¹⁶ to 8 × 10¹⁷ ions/cm² were implanted in Ti foil of 1.8 mm thickness. X-ray diffraction (XRD) was used to obtain the structural characteristics, while atomic force microscope (AFM) was employed to obtain the surface morphology of the samples. The potentiodynamic method was employed to obtain corrosion resistance of the samples in NaCl (3.5%) solution. Titanium nitride formation was enhanced with increasing the nitrogen ion flux, while grain size and surface roughness of the samples were also increased. Optimum corrosion resistance was obtained for 5 × 10¹⁶ (N⁺ ions/cm²).     

Mechanisms of self-lubrication in patterned TiN coatings containing solid lubricant microreservoirs

The tribological mechanisms of friction and lubrication have been investigated in TiN coatings patterned to contain microscopic reservoirs for solid lubricant entrapment. Photo-lithography was used to fabricate three sets of samples on silicon wafers, varying the reservoir size (4 and 9 μm) and spacing (11 and 25 μm), which resulted in samples with a nominal reservoir area of either 2 or 10%. Pin-on-disk tests were run using lubricants of graphite and indium and counterfaces of alumina and steel (440C). In most cases, the samples with the 9 μm holes spaced 25 μm apart gave the lowest friction coefficients and longest wear life. Analysis of the wear tracks by SEM/EDS methods showed carbon to be present in the holes of the graphite/steel counterface samples, but TiO₂ was found in the holes of the graphite/alumina counterface samples. For the indium/steel counterface samples indium was detected within the microreservoirs, but iron was also found, transferred from the ball. These experiments highlight a variety of tribological mechanisms that can operate in microreservoir-patterned coatings.     

喷丸强化对H13钢表面完整性的影响

为研究不同强度喷丸对H13钢表面完整性的影响,采用白光干涉仪、显微硬度计、XRD、SEM和EBSD等对喷丸前后H13钢试样的表面粗糙度、硬度、残余应力和表层微观组织等表面完整性进行了表征,并定量分析了未喷丸和经0.33A喷丸的试样表层组织的晶粒尺寸、晶界取向差及织构等的变化规律。结果表明,不同强度喷丸均改变了H13钢的表面完整性。喷丸在提高表面粗糙度和塑性硬化程度的同时引入了具有一定深度的残余压应力层。与未喷丸试样相比,经0.23 A喷丸后H13钢表面粗糙度提高了约152%。随着喷丸强度从0.23 A增大到0.33 A,硬化层深度由100μm增至160μm,残余压应力层深度由200μm增至300μm。H13钢的未喷丸组织的平均晶粒尺寸为950 nm,晶界平均取向差为33.5858°,主要存在强度较弱的{001}100和{110}111型混合织构;经0.33 A喷丸后距表面10μm处的平均晶粒细化至470 nm,晶界平均取向差增至39.0228°,组织中出现了两类{111}uvw和{hkl}110型板织构,且表层晶粒细化层深度达30μm以上。     

Effects of superimposed high-frequency vibration on deformation of aluminum in micro/meso-scale upsetting

Micro/meso forming is an economically competitive process for the fabrication of miniature metallic parts. However, the size effects observed when scaling down the process leads to challenges such as tribological problems at the interface and premature tool wear due to localized stress concentration regions. In this study, a hybrid micro/meso forming assisted by high-frequency vibration and the contributing mechanisms of the high-frequency vibration were investigated. The effects of high-frequency vibration on the improvement of surface finish, decrease of the friction at the die-specimen interface, and reduction of forming stress were analyzed and discussed based on the vibration-assisted upsetting experiments. In addition, finite element analysis was conducted to help understand the significance of the vibration. Results show that a transverse vibration of 9.3 kHz led to surface roughness (Ra) reduction from 1.5 μm to 0.9 μm at the top surface, friction coefficient decrease from 0.14 to 0.07 at the punch/specimen interface, and nearly 50% reduction in the forming stress. The findings of this study confirmed the significant benefits of high-frequency vibration applied in micro/meso forming of metallic materials and provided a basis to understand the underlying mechanisms of vibration-assisted forming.     

Surface modification of poly(tetrafluoroethylene) by saddle field fast atom beam source

The surface of poly(tetrafluoroethylene) (teflon, PTFE) was treated by saddle field fast atom beam (FAB) source in hydrogen, helium and nitrogen with about 1 kV accelerating voltage, up to an estimated particle fluence of 10¹⁷ cm^− 2. The untreated and FAB-treated samples were characterised by XPS, Raman microspectroscopy, single pass topographic and multipass wear tests and water contact-angle measurements. According to XPS results, upon FAB-treatment the surface F/C value decreased drastically. Raman microspectroscopic measurements testified to the formation of a carbonised surface layer. The thickness of the graded, amorphous carbon-containing layer determined by in-depth Raman microspectroscopic analysis ranged between 5.5 ± 1 and 10.5 ± 1 μm. In addition to radiation damage, a decisive role of degradation induced by thermal effects and diffusion of reactive particles was suggested. The small scale wear resistance of the hydrogen and nitrogen FAB-treated samples improved in comparison with that of the untreated PTFE. The mean surface roughness increased for the treated samples in the order of N < He < H. Water contact-angle decreased upon FAB-treatment. The effect of increased wettability remained durable for at least three months.     

Friction behaviour of diamond-like carbon films with varying mechanical properties

Diamond-like carbon (DLC) films deposited on silicon wafer with varying film thickness were investigated for their micro-scale friction behaviour. Films with three different thicknesses, namely 100 nm, 500 nm and 1000 nm, deposited by a radio frequency plasma-assisted chemical vapor deposition method on Si (100) wafer, were used as the test samples. The elastic modulus of the DLC samples increased with their film thickness. The micro-scale friction tests were conducted in a ball-on-flat type micro-tribotester, using soda lime glass balls with different radii (0.25 mm, 0.5 mm and 1 mm), and with varying applied normal load (load range: 1500 μN to 4800 μN). Results showed that the friction force increased with applied normal load, whereas with respect to the ball size, two different trends were observed. In the case of 100 nm thick sample, friction increased with the ball size at any given normal load, while for 500 nm and 1000 nm thick samples, friction had an inverse relation with the ball size at all applied normal loads. The friction behaviour in the case of the 100 nm thick film was adhesive in nature, whereas for the thicker films plowing was dominant. The friction behaviour of the test samples with the ball size, which was distinctly different, was discussed in terms of the contact area, influenced by their mechanical property, namely, the elastic modulus.     

A comparative study on titania layers formed on Ti, Ti-6Al-4V and NiTi shape memory alloy through a low temperature oxidation process

For improving the bioactivity and biocompatibility of metals for medical applications, anatase titania layers were synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using the H₂O₂-oxidation and hot water aging treatment method at 80 °C. The thickness of the titania layers on Ti, Ti-6Al-4V and NiTi SMA was 7.43 ± 0.93 μm, 3.14 ± 0.38 μm and 4.04 ± 0.25 μm, respectively. X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis indicated that the titania layers formed were poorly crystalline anatase. Fourier transform infrared spectroscopy (FTIR) suggested that abundant Ti-OH functional groups were produced on titania, which could improve bioactivity of the metals. In addition, the titania layer formed on Ti substrate was shown to contain more molecularly chemisorbed water and Ti-OH functional groups than those on Ti-6Al-4V and NiTi SMA. Atomic force microscopic (AFM) results showed that the surface roughness values of metal samples depended on the scanning size and that surface roughness of samples significantly increased after the H₂O₂-oxidation and hot water aging treatment for all three metals. Compared to Ti-6Al-4V and NiTi SMA, the H₂O₂-treated and aged Ti samples exhibited the roughest surface. The wettability of samples was evaluated through water contact angle measurements. After the H₂O₂-oxidation treatment, the three metals exhibited high hydrophilicity. The bonding strength of titania layers on Ti, Ti-6Al-4V and NiTi was also investigated. Potentiodynamic polarization tests indicated that the corrosion resistance of H₂O₂-treated and aged Ti, Ti-6Al-4V and NiTi SMA was significantly improved due to the titania layer formation.