Yuquan Ding

Effect of grain size on friction and wear of nanocrystalline aluminum

Abstract The friction and wear characteristics of nanocrystalline aluminum were investigated as a function of grain size. Nanocrystalline aluminum samples with an average diameter of 16.4 nm were produced using an r.f. magnetron sputtering technique. The grain size was increased (up to 98.0 nm) by an isothermal annealing treatment at 573 K. Hardness measurements were performed using an ultra-microhardness indentation system and it was observed that within the grain size range of 15–100 nm the hardness-grain size data could be well represented by the Hall-Petch relationship. Friction and wear measurements were made using a miniature pin-on-disk type tribometer under unlubricated conditions both in air and in vacuum. The coefficient of friction of aluminum tested against a stainless steel pin varied with the sliding distance. At the early stages of sliding the coefficient of friction rose to a peak value, and this was followed by a decrease to a steady-state value. The transition on the friction curve corresponded to a similar transition from a severe wear regime to a mild wear above a characteristic sliding distance on the cumulative volume loss versus sliding distance curve. The value of the peak coefficient of friction decreased from μp = 1.4 for aluminum with a coarse grain size (106 nm) to μp = 0.6 for the nanocrystalline aluminum with a grain size of 16.4 nm. The coefficient of friction of nanocrystalline aluminum showed a 30% increase when tested in vacuum. In the nanocrystalline grain range, the wear rates were found to be linearly dependent on the square root of the grain size. An empirical equation based on the Archards Law is proposed to describe the effect of grain refinement on the wear resistance under unlubricated sliding conditions. A qualitative understanding of wear processes is developed in terms of the variation of the surface morphology and subsurface strength with sliding distance.

Nanoindentation and friction studies on Ti-based nanolaminated films

Abstract Multilayered Ti/TiN and Ti/Cu films of 12–15 μm thickness were produced using an r.f. triode-magnetron sputtering technique. The thickness of Ti layers in both types of composites varied between 150 and 1000 nm. TiN and Cu layers were thinner, and their thicknesses varied within a narrower range of 20–120 nm. The volume fractions of the constituents in the composites were kept constant in order to study the effect of reducing Ti layer thickness (λTi) on the mechanical and tribological properties of the laminates. Nanoindentation tests were performed to determine hardnesses and elastic moduli of the composites and to analyze the energy expenditure during the indentation process. The elastic modulus and hardness of the Ti/TiN films were both higher when tested in a direction parallel to the plane of the layers compared to a direction normal to their planes. The strength of Ti/TiN also increased with (λTi) and showed a good agreement with the strength levels calculated by considering the difference in the elastic moduli of the constituents on the basis of the Koehler strengthening mechanism. Thin Cu layers in Ti/Cu were not effective as barriers to dislocation motion, and Ti/Cu interfaces were susceptible to delamination during indentation tests and sliding wear tests. A linear relationship was observed between the wear rates of Ti/TiN and (λTi)−0.5 which suggests that the wear resistance of the laminated composites can be improved by reducing the distance between the TiN layers.

PVD NiAl intermetallic coatings : microstructure and mechanical properties

Abstract NiAl intermetallic coatings were fabricated using an rf magnetron sputtering system and a specially-designed composite target made from pure Ni and Al metals. The coatings which were 10 μ m thick were deposited on a variety of substrates including 1100Al, commercial purity nickel, 316 type stainless steel and borosilicate glass. X-Ray diffraction (XRD) showed that the coatings produced for all substrates were the NiAl intermetallic compound with the ordered cubic B2 crystal structure. The coating deposited on a borosilicate glass substrate had a (111) preferred orientation. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed that the grain size of the NiAl was 10 nm. The hardness and elastic modulus of the NiAl coatings were measured using an ultra-microhardness indentation system and found to be 11.52 GPa and 143.42 GPa, respectively. The NiAl intermetallic coating worn against a steel bearing pin exhibited a low wear rate (2.9 × 10 −4 mm 3 /m). The NiAl coating had a lower value of coefficient of friction (0.23) than the aluminum substrate (0.28) as measured using a scratch tester.

Fabrication by magnetron sputtering of Al-Cu-Fe quasicrystalline films for tribological applications

Abstract Al-Cu-Fe (Al 65 Cu 20 Fe 15 ) films were fabricated using an RF triode magnetron sputtering technique. The films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) techniques. The as-sputtered Al-Cu-Fe films had an amorphous structure. The Al-Cu-Fe film annealed at 750°C for 2 h had an icosahedral quasicrystalline structure, but annealing at temperatures from 200°C to 550°C produced a mixed structure of amorphous and quasicrystalline phases. Nanoindentation measurements showed that the hardness and elastic modulus of the as-sputtered Al-Cu-Fe films (9.44 GPa and 114.4 GPa, respectively) increased as the amount of the quasicrystalline phase increased on annealing. The Al-Cu-Fe film annealed at 550°C for one hour had a hardness of 13.49 GPa and a Youngs modulus of 141.4 GPa. A systematic increase in wear resistance with increasing annealing temperature was observed, based on the scratch track width measurements. Both the as-sputtered (amorphous) and annealed (amorphous + quasicrystalline) Al-C-Fe films had low coefficients of friction (0.09–0.11) that were of the same order as for TiN films produced by reactive ion plating (RIP).

Mechanical properties and tribological behaviour of nanolayered Al/Al2O3 and Ti/TiN composites

Mechanical and tribological properties of nanolayered composites of Al/Al 2 O 3 and Ti/TiN were investigated. Alternating layers of metals and ceramics were deposited using an r.f. magnetron sputtering technique. Nanoindentation tests were performed to determine force-displacement curves which were used to calculate elastic moduli and nanohardness of composites as a function of distance between layers. It was observed that both elastic modulus and hardness of composites increased with decreasing layer thickness. A good agreement was found between experimentally determined values for elastic modulus and predictions based on the rule of mixtures for isostress conditions. The hardness of Al/Al 2 O 3 and Ti/TiN could be described in the formalism of the Hall-Petch-type equation indicating that ceramic layers inhibited slip transfer across metallic layers. A deviation from the Hall-Petch type of strengthening was observed in Al/Al 2 O 3 at small interlayer spacings. Friction and wear behaviour of laminates was studied using a pin-on-disc type of tribometer. A systematic increase in wear resistance with decreasing layer thickness was observed. The peak friction coefficient decreased about 70% in Al/Al 2 O 3 (with 200 nm Al layer thickness) while a significant 60% improvement in steady state friction coefficient was measured in Ti/TiN (with 150 nm Ti layer thickness) in comparison with as-sputtered monolithic metallic films. These observations indicated that the nanolaminated films are suitable for applications where a combination of low coefficient of friction, high wear resistance and hardness is required

A study of the interphase structure at the oxide-metal interface in the corrosion of zirconium-based alloys

Abstract The nature of the oxide layer that formed during corrosion of zirconium-based alloys in a pressurized, lithiated water environment at 300°C was determined through scanning electron microscopy (SEM) study of the oxide-metal interface region as well as X-ray diffraction (XRD) and electron diffraction (ED) analyses. Alloys examined included cold-worked Zr-2.5wt%Nb, heat-treated Zr-2.5wt%Nb, Excel alloy (Zr-Sn-Mo-Nb), pure zirconium and cold-worked Zircaloy-4 (Zr-Sn alloy). The metallographic observations and corrosion kinetics studies show that variations in the oxidation rate among the zirconium-based alloys result from the differences in structure of the interphase layer at the oxide-metal interface.

Fabrication of microlaminated Al/Al2O3 composites by magnetron sputtering for tribological applications

Abstract Microlaminated composites consisting of alternating layers of Al and Al 2 O 3 have been produced using a radiofrequency triode magnetron sputtering unit equipped with metallic and oxide target sources. Composite films up to 25 μm thick with 100 Al/Al 2 O 3 layers were fabricated in the form of square coupons 20 mm × 20 mm, which were suitable for wear testing using a miniature pin-on-disc tribometer. To measure the strength and the wear resistance as a function of the interlaminate spacing, composites with Al layers 100–500 nm thick were fabricated. The thicknesses of the Al 2 O 3 layers were in the range 20–100 nm. Transmission electron microscopy and selected area electron deffraction studies showed that every Al layer consisted of polycrystallites with very fine grain size (20 nm on a average), whereas the Al 2 O 3 layers had an amorphous structure. Microhardness measurements showed that monolithic nanocrystalline Al produced using sputtering had a hardness of 1700 MPa (eight times higher than that of a pure Al target with a grain size of 5 mm) and the incorporation of Al 2 O 3 layers increased the microhardness to 2300 MPa. Tribological tests revealed that the dry sliding wear resistance of Al/Al 2 O 3 composites increased by a factor of two compared with that for monolithic nanocrystalline Al.

Tem study of the oxide-metal interface formed during corrosion of Zr-2.5wt.%Nb pressure tubing

This paper reports on a specimen preparation technique which has been developed to examine the oxide-metal interface in Zr-2. 5wt. % NB pressure tubing corroded in pressurized water at 300[degrees]C. In order to avoid any chemical damage of the oxide-metal interface, an ion-beam milling technique was used, an no acids or other corrosive solutions were used during specimen preparation. Examination of the thin-foil specimens using transmission electron microscopy imaging and selected-area electron diffraction techniques showed that the oxide-metal interface was tetragonal ZrO[sub 2], whereas the oxide at the oxide-corrodent surface and away from the oxide-metal interface was monoclinic ZrO[sub 2]. Differences were noted in the structure of the oxide formed over the [alpha] and [beta] phases of the metal. The retention of tetragonal ZrO[sub 2] at the oxide-metal interface is attributed to the fine grain size of the ZrO[sub 2] and the compressive stresses developed at the oxide-metal interface.

The processing and testing of new and advanced materials for wear resistant surface coatings

Abstract In the search of wear resistant coatings for applications such as cutting tools and turbine refurbishing, we have produced and tested a wide range of coatings including nanocrystalline metals (At, Ti, Cu), nanolaminated composites, containing alternating layers of metal-ceramic (Al 2 O 3 and Ti/TiN) or metal-metal (Ti/Cu), and monolithic coatings (TiN). These coatings were produced using an r.f. magnetron sputtering system except for the monolithic TiN which was commercially produced using an ion plating system. All these coatings were tested for their tribological (friction and wear) and mechanical (hardness) properties using a pin-on-disc type wear rig and an ultra-microindentation system, respectively. The tribological properties that were used to compare the relative performance of these materials were the coefficients of friction (both peak and steady-state) and the wear rates (volume loss per unit sliding distance) in both the severe and mild wear regimes. The optimum combination of a low coefficient of friction and low wear rate in the steady-state regime was exhibited by Ti/TiN nanolaminated composite coating with a 150 nm Ti layer thickness and a 20 nm TiN layer thickness. This material also exhibited the highest hardness of the composite coatings. The micromechanisms responsible for the differences in hardness and wear resistance of these materials are discussed and recommendations are made on future directions for producing improved wear resistant coatings.

Methods for preparation of cross-sectional scanning electron microscopy specimens and their application to corroded specimens of a zirconium alloy and TiN coated stainless steel

Abstract Two techniques for the preparation of cross-sectional scanning electron microscopy specimens are described. The method that utilizes a tapered cross section is emphasized. This method was developed for the examination of the oxide-metal interface in zirconium alloy corrosion specimens. Typical micrographs are presented for the oxide-metal interface in Zr-2.5wt.% Nb alloy corrosion specimens and for the TiN-substrate interface for TiN coated stainless steel.