The mechanisms of formation of ohmic contacts to AlGaAs: A microstructural,elemental diffusion and electrical investigation

Interfacial microstructure and phase composition of PtTiGePd ohmic contacts to heavily C-doped AlGaAs were investigated as a function of annealing temperatures. Results of the material analyses were used to explain the specific contact resistances measured for each thermal treatment. Evidence of interdiffusion and compound formation between AlGaAs and Pd was visible in a Ga-rich Pd-Ga-As reaction zone prior to heat treatment. As the annealing temperature was raised from 530 to 600°C, As began to out-diffuse. At 600°C, this As out-diffusion, which is critical to the formation of good p-type ohmic contacts, contributed to the creation and development of the laterally continous two-phase interfacial region, TiAs/Pd₁₂Ga₂Ge₅, overlying the AlGaAs substrate. The minimum specific contact resistance was also achieved at this temperature. As the annealing temperature was elevated to 650°C, the specific contact resistance degraded in response to intensive chemical diffusion and development of a broad, nonuniform multiphased interfacial region.     

Evolution of rapidly solidified NiAlCu(B) alloy microstructure

This study concerned phase transformations observed after rapid solidification and annealing at 500, 700 and 800 °C in 56.3 Ni‐39.9 Al‐3.8 Cu‐0.06 B (E1) and 59.8 Ni‐36.0 Al‐4.3 Cu‐0.06 B (E2) alloys (composition in at.%). Injection casting led to a homogeneous structure of very small, one‐phase grains (2–4 µm in size). In both alloys, the phase observed at room temperature was martensite of L1₀ structure. The process of the formation of the Ni₅Al₃ phase by atomic reordering proceeded at 285–394 °C in the case of E1 alloy and 450–550 °C in the case of E2 alloy. Further decomposition into NiAl (β) and Ni₃Al (γ′) phases, the microstructure and crystallography of the phases depended on the path of transformations, proceeding in the investigated case through the transformation of martensite crystallographic variants. This preserved precise crystallographic orientation between the subsequent phases, very stable plate‐like morphology and very small β + γ′ grains after annealing at 800 °C.     

Metallurgical interactions at the contacts GaAs interface

Interface morphology, phase composition, and elemental diffusion of Pt/Ti/Ge/Pd ohmic contacts to both n and p⁺-GaAs have been investigated as a function of annealing temperature. Structural and chemical results were correlated with specific contact resistances (ρ_c) measured for each thermal treatment. Annealing at 450°C yielded the lowest ρ_c, ～6.4 × 10^?7Ω-cm². The interface was observed to be smooth and abrupt. Two interface phases were detected; a primary phase, PdGe, and a secondary, Ga-rich Pd-Ga-As ternary phase. The presence of this ternary phase was found to be critical to contact formation on n-GaAs. The Ti and Pt layers remained pristine. Annealing at 550°C resulted in a slightly higher ρ_c, ～2.1 × 10^?6Ω-cm². There was significant elemental diffusion within the contact metal and minor diffusion into the GaAs substrate. The interface possessed limited areas of spiking with uniform composition. Annealing at 600°C proved to have a detrimental effect on the ρ_c, ～10^?4Ω-cm². This electrical degradation was accompanied by strong chemical intermixing between the contact and substrate, resulting in a continuous nonplanar interface with deep multiphase protrusions.     

Dependence of the sizes of damaged regions around indium contacts to p-type CdHgTe on GaAs substrates on the annealing temperature and time

It is found that damaged regions are formed around indium contacts to p-type CdHgTe {310} heteroepitaxail layers (HELs) on GaAs substrates and the sizes of these regions depend on the temperature and time of annealing in air. It is shown experimentally that at an annealing temperature of 90 °C, the rate of expansion of the damaged regions is about 4 µm/h, and at temperatures of 120 °C, it is more than 25 µm/h. After 488 hours of annealing of plates of CdHgTe HELs at 60 °C in air, the formation of damaged regions around the indium contacts to the p regions was not observed. The studies were performed on plates of p-type CdHgTe HELs on GaAs substrates whose surface was covered with SiO₂ and Si₃N₄ dielectrics (with a total thickness of about 0.15 µm), with windows where p-n junctions were generated by ion implantation of boron.     

Mild wear of a low alloy steel at temperatures up to 500°C

Wear behaviour of 52100 low alloy steel has been studied on a pin on disc wear machine at disc temperatures ranging from room temperature to 500°C. Transitions occur in the wear rate versus load curves at certain critical loads, the magnitude of which increase with temperature. These transitions were found to be associated with change in surface oxide, lower wear rates being recorded when the predominant oxide was the spinel Fe₃O₄ for all temperatures. At disc temperatures above 300°C out of contact oxidation appears to be the most important wear limiting factor. A surface model was developed enabling contact temperature, numbers and size of contacts and critical oxide film thickness to be deduced. Remarkable agreement was found between oxide thicknesses estimated from this model and measured values using a scanning electron microscope     

Effect of temperature on the fretting corrosion of tin plated copper alloy contacts

The effect of temperature on the fretting corrosion behaviour of tin plated copper alloy contacts in the temperature range of 25–185 °C, is addressed in this paper. The change in contact resistance with fretting cycles at various temperatures was determined. The contact zone after fretting corrosion test was analyzed using laser scanning microscope, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray spectrometry (EDX), to assess the surface profile, phase content, morphology and compositional changes across the interface. The study reveals that temperature has a greater influence on the extent of fretting corrosion of tin plated copper alloy contacts. The softening of tin is responsible for the extended region of low contact resistance observed at 85 °C. The increase in thickness and the resistance of Cu–Sn intermetallic compounds (IMCs) is responsible for the decrease in surface roughness and the drastic increase in the contact resistance at higher temperatures. The study suggests that the tin plated copper alloy contact system should be considered as copper alloy/IMC/Sn/SnO₂ instead tin plated copper alloy. During fretting corrosion test at elevated temperatures, once the top surface layers are worn out, the contact interface is transformed from tin versus tin-to-tin-intermetallic versus tin-intermetallic. The study concludes that tin plated copper alloy contacts are not suitable for high temperature applications.     

TEM studies of the nitrided Ni-Ti surface layer

The structure of surface layer, obtained on the nearly equiatomic Ni‐Ti alloy after nitriding under glow discharge conditions at temperatures 700 or 800 °C, was investigated. The structural characterization of the intruded layer was performed on cross‐sectional thin foils by the use of the transmission and scanning electron microscopes. The obtained results show that the nitrided layers consist mainly of the nanocrystalline TiN phase and small amount of Ti₂N. Between the nitrided layers and β‐NiTi matrix an intermediate Ti₂Ni phase layer was observed.     

Electron microscopic studies of CuS nanocrystals formed in Langmuir-Blodgett films

The morphology and structure of CuS crystals formed during sulfidation of copper behenate films obtained by the Langmuir-Blodgett (LB) method have been studied using high resolution electron microscopy. The average size of these crystals is about 3 nm and increases by a factor of approximately 2.2 after annealing at a temperature of 150 °C or above. Analysis of interplanar distances has shown that in the range of annealing temperatures of 150–200 °C, CuS nanocrystals have a P6_3/mmc hexagonal crystal lattice with parameters a = 0.38 nm and c = 1.64 nm. At annealing temperatures of 250 °C or above, the Cu₂S crystalline phase begins to form, in addition to CuS nanocrystals. The proportion of this phase increases with increasing annealing temperature. Cu₂S nanocrystals have a hexagonal crystal lattice type with the P6_3/mmc spatial group and unit cell parameters a = 0.39 nm and c = 0.68 nm. Quantitative evaluation of copper and sulfur in individual CuS and Cu₂S nanocrystals was performed by local analysis of characteristic X-ray spectra.     

The characteristics of intermetallic compounds and hardness of Ni-Cr/AC8A composites under different infiltration temperatures

With the rapid development of aerospace and automobile industries, there is an increasing need for structural materials with excellent mechanical properties such as can be used at high temperatures. Aluminum matrix composite have attracted much attention because of its excellent performance, particularly the existence of the high temperature resistance inter-metallic compounds. Aluminum-based composites with inter-metallic compounds have been studied recently. In this study, Ni-Cr porous preform is reinforcement when Al-alloy (AC8A) is base metal and Ni-Cr/AC8A composites were manufactured under low pressure infiltration limited to the maximum of 0.5 Mpa at 700°C, 750°C, 800°C and 850°C, respectively. The microstructure and phase composition of the composites were evaluated by optical microscope, X-ray diffraction (XRD) and electro-probe micro analyzer (EPMA), intermetallic compounds Al₃Ni and CrSi were found as newly formed phases in the composites. In addition, Vickers hardness of Ni-Cr composites was also tested.     

On the friction mechanism of solid lubricants

It is shown that the coefficient of friction of polycrystalline graphite materials is governed by the crystallite size and adsorption heat of the molecules making up the lubricating layer. In the absence of a lubricating layer in the actual contact zones (in vacuum, dried gases, and in air at temperatures above 200°C), graphite loses its lubricity. To maintain low friction within 800°C, ultraphosphates are introduced into graphite pores; in heating, they release vapors of phosphoric anhydride to form a lubricating film. At 300°C, chalcogens MoS₂, WS₂, and MoSe₂ release S and Se vapors that are adsorbed in contact and create a lubricating film. The capacity to release vapors is governed by the presence of solid solutions of chalcogens in addition to stoichiometric compounds.     

Tribo-oxidation of Self-mated Ti3SiC2 at Elevated Temperatures and Low Speed

The tribological behavior of self-mated Ti₃SiC₂ is investigated from ambient temperature to 800?°C at a sliding speed of 0.01?m/s in air. The results show that at the temperatures lower than 300?°C, friction coefficient and wear rates are as high as 0.95 and 10^?3?mm³/N?m, respectively. With the temperature increasing to 600?°C, both the friction coefficient and wear rates show consecutive decrease. At 700 and 800?°C, friction coefficient and wear rates are 0.5 and 10^?6 mm³/N?m, respectively. According to the wear mechanism, the tribological behavior of Ti₃SiC₂ can be divided into three regimes: mechanical wear-dominated regime from ambient temperature to 300?°C characterized by pullout of grains; mixed wear regime (mechanical wear and oxidation wear) from 400 to 600?°C; and tribo-oxidation-dominated wear regime above 700?°C. The tribo-oxides on the worn surfaces involve oxides of Si and Ti. And, species transformation occurs to these two oxides with the increasing temperature. In the competition oxidation of elements Ti and Si, Si is preferably oxidized because of its high active position in the crystal structure. Additionally, plastic flow is another notable characteristic for the tribological behavior of self-mated Ti₃SiC₂.     

Zinc oxythiomolybdate (ZnMoO2S2) powder as a solid lubricant for high-temperature rolling contact applications

Conventional liquid lubricants being used in today's gas turbine engines will not be able to operate effectively in the hostile bearing environments expected in future turbine engines. The expected high operating temperatures (500–800°C) mandate new and innovative lubrication schemes to achieve success. Recent studies have demonstrated that a new class of solid lubricants, the complex chalcogenides or metal ocythiomolybdates, have good potential for high temperature lubrication. This paper describes the friction, wear and rolling contact endurance of three high-temperature bearing materials using a zinc oxythiomolybdate (ZnMoO₂S₂) powder lubricant. Rolling contact tests were conducted using VIM-VAR M50, micromelt T15 tool steels and silicon nitride (Si₃N₄) at temperatures ranging from 23°C to 649°C, using a modified ball-on-rod type rolling-contact fatigue tester. Significant improvements in friction, endurance and wear were observed at all test temperatures, and with all three materials evaluated, when ZnMoO₂S₂ was used as a lubricant. Overall, silicon nitride exhibited the best frictional and antiwear performance. The lubricant powder exhibited the best tribological performance with T15 and M50 specimens between 177°C and 316°C. Energy Dispersive X-Ray Analysis (EDAX) of wear tracks showed the presence of iron (Fe) on the Si₃N₄ specimens as well as the presence of zinc (Zn) on both the T15 and the M50 specimens.     

The optimization of annealing and cold-drawing in the manufacture of the Ni�CTi shape memory alloy ultra-thin wire

In this paper, the mechanical properties of the Ni?C50.5?at.%?CTi alloy super-elastic wires manufactured by a conditioned multi-passed process of annealing and cold-drawing have been studied. The annealing temperature of 450~800°C, time of 20?min~3?h and the cold-drawing amount of 6.9%~39% were chosen. Their effects on the thermo, mechanical, and surface morphology of the Ni?CTi wires have been studied. The differential scanning calorimetry and tensile-recovery tests were adopted to obtain the phase transformation temperatures and mechanical hysteresis of the Ni?CTi SMA wires with different treatment conditions. The results show that the phase transition temperature of Ni?CTi wire can be changed by varying the annealing temperature and time; cold-drawing deformation and subsequent annealing have a great influence on the super-elasticity. The process with 39% cold-drawing amount, 600°C and 20?min annealing is shown to be effective in the manufacturing.     

Effects of ZnO and MoS<Subscript>2</Subscript> Solid Lubricants on Mechanical and Tribological Properties of M50-Steel-Based Composites at High Temperatures: Experimental and Simulation Study

Prospective beneficial effects of mixtures of temperature-adaptive solid lubricants (ZnO–MoS₂) on mechanical and tribological properties of M50 alloy steel were investigated at temperatures from 25 to 800 °C. ZnO and MoS₂ were mixed with M50 (designated as M) to create composites MZ (M50 steel plus ZnO), MM (M50 steel plus MoS₂), and MZM (M50 steel plus both additives). Sliding friction and wear experiments were performed at different temperatures using a pin-on-disk at a sliding speed of 0.2 m s^?1 and a load of 12 N. Silicon nitride and M50 steel were used as the pin materials. In order to understand the friction and wear behavior of composites, analyses of their surfaces were done using XRD, EPMA, FESEM, EDS line/mapping, and XPS tests. A dynamic simulation model based on the finite element method was built to simulate the different stresses on the contact pairs. Results elucidated that MZM attained the least friction (0.17), compared to M (0.40), MZ (0.26), or MM (0.29) at 800 °C. The increase in surface roughness of MZM due to sliding was reduced by 37.3% compared to that of MZ (11.9%) or MM (22.7%). The good lubricating behaviors were referred to the synergetic effects of ZnO, MoS₂, and formed lubricating components on worn surfaces.     

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS₂ (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti₃SiC₂ (a high-temperature lubricant, above 600°C) designated as MhT against an Si₃N₄ counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS₂, hBN, and Ti₃SiC₂. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.     

Adaptive Mo2N/MoS2/Ag Tribological Nanocomposite Coatings for Aerospace Applications

Reactively sputtered Mo₂N/MoS₂/Ag nanocomposite coatings were deposited from three individual Mo, MoS₂, and Ag targets in a nitrogen environment onto Si (111), 440C grade stainless steel, and inconel 600 substrates. The power to the Mo target was kept constant, while power to the MoS₂ and Ag targets was varied to obtain different coating compositions. The coatings consisted of Mo₂N, with silver and/or sulfur additions of up to approximately 24 at%. Coating chemistry and crystal structure were evaluated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which showed the presence of tetragonal Mo₂N and cubic Ag phases. The MoS₂ phase was detected from XPS analysis and was likely present as an amorphous inclusion based on the absence of characteristic XRD peaks. The tribological properties of the coatings were investigated in dry sliding at room temperature against Si₃N₄, 440C stainless steel, and Al₂O₃. Tribological testing was also conducted at 350 and 600 °C against Si₃N₄. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry, energy dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy. During room temperature tests, the coefficients of friction (CoF) were relatively high (0.5–1.0) for all coating compositions, and particularly high against Si₃N₄ counterfaces. During high-temperature tests, the CoF of single-phase Mo₂N coatings remained high, but much lower CoFs were observed for composite coatings with both Ag and S additions. CoF values were maintained as low as 0.1 over 10,000 cycles for samples with Ag content in excess of 16 at% and with sulfur content in the 5–14 at% range. The chemistry and phase analysis of coating contact surfaces showed temperature-adaptive behavior with the formation of metallic silver at 350 °C and silver molybdate compounds at 600 °C tests. These adaptive Mo₂N/MoS₂/Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo₂N and Mo₂N/Ag coatings, hence providing a high potential for lubrication and wear prevention of high-temperature sliding contacts.     

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT

The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr₃C₂-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.     

Microstructural and Tribological Characterization of Stainless Steel–Reinforced Aluminum Matrix Bimetal Composites Produced at Various Mold Burnout Temperatures

This study aims to identify the optimal burnout temperature (BT) of a plaster mold that was used in bimetal composite production. To achieve this goal, the mold was gradually heated up to 600, 650, 700, and 750?°C prior to melt infiltration casting. Molten A356 aluminum alloy was cast into mold at 730?°C for each casting process. Fifty percent porous 304 stainless steel (SS) preforms, obtained by assembling recycled SS shavings, were placed in a mold and infiltrated by A356 alloy until solidification was completed. The produced bimetal composites were subjected to a ball-on-disc tribometer with loads of 5, 10, and 15 N for 100 m sliding distance using an Al₂O₃ ball as a counterpart. θ-Fe₄Al₁₃ and η-Fe₂Al₅ phases were formed at A356 Al–304 SS interfaces for all samples. Wear rates increased with increasing load and decreased with increasing BT, except at 750?°C. At this temperature, interfacial phases with excessively increased layer thickness, hardness, and brittleness were fragmentized during the test, and these cracked particles decreased wear resistance by participating in the wear process. The most suitable BT of the mold was found to be 700?°C, considering the microstructure and wear results of bimetal composites.     

Effects of wear speeds on friction-wear behaviours of cathode arc ion plated TiAlSiN coatings at high temperatures

A TiAlSiN coating was deposited on AISI H13 hot work mould steel using a cathodic arc ion plating (CAIP). The microstructures, chemical composition and phases of the obtained coatings were analysed using a field emission scanning electronic microscope, energy dispersive spectrometer (EDS) and X-ray diffractometer, respectively. The high temperature friction-wear properties of TiAlSiN coating at the different wear speeds were investigated, and the wear mechanism was also discussed. The results show that the N of the TiAlSiN coating is not completely released at 800 °C, the diffraction peak of TiN still exists in this coating. In addition, the products of SiO₂ and Al₂O₃ play a role of self-lubricating and wear resistance. The average coefficient of friction (COF) of the coatings at the wear speeds of 400, 600, and 800 r/min is 0.15, 0.22, and 0.17, respectively. The wear mechanism of TiAlSiN coating at 800 °C is primarily adhesive wear, accompanied by oxidation wear and abrasive wear.     

Low-temperature bitumen stiffness and viscous paraffinic nano- and micro-domains by cryogenic AFM and PDM

In an effort to better understand the structure and behaviour of bitumen in low temperature, we describe the first use of cryogenic atomic force microscopy and phase detection microscopy to characterize bitumen nano‐ and micro‐structures. The results were interpreted in light of glass transition temperatures (T_gs) for bitumen fractions. The domains visible by microscopy, the catana, peri and para phases, were attributed to domains rich in asphaltenes, naphthene and polar aromatics, and saturates, respectively. Between ?10°C and ?30°C, atomic force microscopy images revealed topographic features not visible in atomic force microscopy images acquired at room temperature. According to phase detection microscopy and T_gs, the features were assigned to viscous unfrozen saturates. Upon cooling to ?72°C, unfrozen domains of 20–400 nm were observed. These domains were found in the paraphase rich in saturates and in the periphase rich in naphthene aromatics and polar aromatics. The findings indicate that new viscous domains form upon cooling to low temperatures owing to phase segregation, and that some bitumens are never entirely rigid in low temperatures.