Key Engineering Materials Vol. 527

Abstract: This paper discusses the effect of advanced sintering technologies – sinter + hot isostatic pressing (HIP) and sinter/HIP on the performance of TiC-based cermets. The performance was evaluated by strength (transverse rupture strength), adhesive wear resistance and durability. Adhesive wear was performed by a turning method (turning of mild steel at low speed) while durability was evaluated by functional testing – wear of tools during blanking of sheet metal. It was found that sinter/HIP ensures a higher positive effect than sinter + HIP in different cycles.

Abstract: Possible increase in fracture toughness of ceramics can be derived from stress induced martensite transformation from tetragonal to monoclinic polymorph of ZrO₂ particles embedded into a bulk ceramic material. The incidence of transformations depends on zirconia particle size: too small particles remain overstabilized and do not experience transformation while too large particle may spontaneously transform at the technological stage of cooling. The critical particle size is, therefore, of primary concern for toughening of intrinsically brittle materials. We give a brief review of the previous results obtained. Then basing on the Gibbs energy expression and taking into account interface surface energy as well as thermal stresses, external loading and elastic interaction of the inclusions we estimate the proper range of particle sizes needed for considerable increase in fracture toughness. We specify general results obtained for the case of yttria stabilized ZrO₂ particles in Al₂O₃- and WC-based ceramics.

Abstract: The aim of the study was to investigate the resistance to rolling contact fatigue of a C:H:W and MoS2/Ti coated gears. The investigation of rolling contact fatigue was realised by means of a gear test rig using FZG PT C/10/90 pitting test. Four material combinations of gears were tested: wheel and pinion uncoated, wheel and pinion coated, wheel coated and pinion uncoated as well as wheel uncoated and pinion coated. The tests were performed using for lubrication mineral gear oil of API GL-5 performance level and 80W/90 viscosity grade. The results indicate that for the coated/coated pair (pinion and wheel coated) and coated pinion/steel wheel pair a significant decrease in the fatigue life compared to the uncoated gears was obtained. The best results were obtained in the case of the uncoated pinion / a-C:H:W coated wheel – even fourfold increase in the fatigue life was observed. This shows a very high potential of application of PVD coatings for gears.

Abstract: Diamond-like carbon (DLC) films were prepared in-situ on top of hard coatings, namely, prior to the DLC deposition, CrN, TiCN and nanocomposite nACo (nc-AlTiN/(a-Si3N4) layers were deposited on WC-Co substrates. The Ra roughness of WC-Co substrates was 0.05 and 0.2 µm. Surface morphology of DLC coatings was investigated by means of profilometry and geometrical parameters Ra, Rsk, Rku and Rz were evaluated. Fretting tests were conducted with 3 mm in diameter alumina balls under 1 and 2 N load, at slip distance of 2 mm and frequency of 2 Hz. Statistical analysis shows significant correlation between Ra and Rz parameters corresponding to initial DLC surface and wear scar surfaces produced by fretting tests after 2400 and 4800 cycles, respectively. This finding indicates a relationship between friction (wear) and Ra and Rz parameters. Positive correlation between the coefficient of friction (COF) and kurtosis Rku indicates that surface flatness is an important factor for optimal friction of an alumina ball against DLC coating.

Abstract: Thin NiP and NiP+SiO₂ coatings were deposited by electroless techniques on aluminum alloy plates. Dry sliding tests against Al₂O₃ counterbody with the ball on plate contact geometry and reciprocal moving were carried out. The results of tribological tests showed that the friction coefficient of the modified coating at loads above 500 mN by 3-4 times, and wear 1.4-2 times lower than for original NiP coatings. It was found that during friction of NiP and NiP+SiO₂ coatings different tribochemical processes are realized. These processes lead to the formation of different by chemical composition of P, Ni and O of secondary structures on the friction surfaces and as result to different tribological properties of the investigated coatings.

Abstract: To study the microstructure and mechanical properties of alumina nanocomposites, Al2O3/2.5 vol.% Ni and Al2O3/10 vol.% ZrO₂ nanocomposites were consolidated by pulsed electric current sintering (PECS). Fracture toughness was found to increase by 13 % and 16 % respectively compared to reference alumina. Hardness increased slightly in Al2O3/Ni because of a fraction of nickel particles under the critical size (2 following the rule of mixtures. By investigating the results, causes of improved mechanical properties were critically evaluated.

Abstract: Fatigue strength is one of the most important mechanical properties. Durability and reliability of car parts is often defined by their fatigue strength, since most of them are loaded with dynamic, repeating or variable loads and the main type of failure is metal fatigue. Fatigue crack usually starts on the metal surface. The interrelationship of the surface layer together with the characteristics of internal metal volume determine the value of fatigue limit and the coefficient level of stresses’ intensity, which is required for the start of fatigue crack. Various hardening methods of surface have a huge impact on fatigue strength of structural materials. The choice of surface processing method is determined by properties and microstructure of a material, as well as the purpose and working conditions of part’s material. Very often the optimum processing is a combination of several methods, which enables to obtain the required properties (high fatigue strength, wear, etc.). Hardening with high-frequency electric current (HfEC) is widely used process for the surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined. Investigation of specific fatigue fracture properties enables to determine the kinetic of processes, related with fatigue crack initiation and propagation. Analysing fractures of machine parts and comparing them with the most specific “mode” fractures it is possible to determine the nature of affecting loads and the reason of part failure. The impact of several combined surfaces processing on the fatigue strength and fractures of carbon steel samples is investigated in the work. The surface was hardened using different processing combinations: by hardening with high frequency electricity currents, rolling by rollers, heating and cooling under different temperatures. Experimentally it was proved that thermal treatment of plastically deformed carbon steel significantly increases fatigue strength. After surface hardening with the given regimes, the microstructure and residual stresses are formed in such way that fatigue crack begins to grow under the hardened sample surface. Samples, in which the fatigue crack initiates inside the sample, have higher fatigue strength than the samples, the fatigue crack of which initiates on the surface.

Abstract: In this paper pulsed electric current sintering (PECS) is applied for submicron-sized copper (sm-Cu) based composite-powders aiming to produce MMC’s with higher strength and better temperature stability than reference sm-Cu. Incorporation of cuprite (Cu₂O), alumina (Al₂O₃), titaniumdiboride (TiB₂) and nano- and submicronsized diamonds (ND’s and SMD’s) improved noticeably the room temperature mechanical properties and the high-temperature stability of copper the effects becoming more noticeable with smaller dispersion size and higher amount of reinforcement. The hardness increment was at highest, when using ND’s or Al₂O₃. E.g., the microhardness for the reference sm-Cu sample and Cu with 3 vol.% ND’s, 6 vol.% ND’s and 2.5 vol.% Al₂O₃ were 1.02, 1.43, 1.77 and 1.58 GPa, respectively. Similar trend was noted also in the case of thermal stability and CTE. The study shows that Cu-ND, Cu-SMD and Cu-Cu₂O are suitable for use at moderate temperatures around 623 - 673 K, whereas Cu-Al₂O₃ and Cu-TiB₂ are suitable above 1023 K. In conclusion, PECS is suitable method to produce high quality Cu-composites having superior thermal and mechanical properties compared to those of sm-Cu.

Abstract: Aluminium 6061 has proven to be a suitable alloy as a basis for producing metal matrix composites (MMC). These MMCs have a low specific weight combined with a relatively good specific stiffness and high specific strength. The hardness and compressive strength of Al composites can be increased by reinforcing bulk material with nano particles. However the ductility of such alloys is relatively low, therefore one of the applications for such light alloys could be wear applications. In many wear conditions such as erosive or abrasive wear at normal impact angles the surface wear resistance plays a significant role. The surface fatigue properties have not been widely studied for such nanoparticle reinforced aluminium composites. The nano-reinforced composite materials were produced by means of high-energy milling (HEM) of nano-sized reinforcement particles together with a metallic matrix powder, followed by hot pressing. By utilizing up to 6 wt% multiwalled carbon nanotubes (MWCNT) as reinforcement the hardness of Al6061 MMC has been increased from 45 HV10 up to 317 HV10, compressive yield strength from 58 MPa up to 660 MPa and indentation modulus from 60 GPa up to 90 GPa compared to hot pressed Al6061. Surface fatigue tests were conducted at impact (dynamic) loading conditions using a hardened steel sphere as indenter. The Wöhler-like curves are plotted to estimate the surface fatigue. The surface fatigue indents were photographed by the aid of light optical microscopy (LOM) and analysed by image analysis software and optical profilometry (OP).