Materials Science Forum Vols. 539-543

Abstract: Oil jet peening is a surface modification process developed for the introduction of compressive residual stresses. In this process, a high-pressure oil jet impinges on the surface to be peened. Specimens made of AISI 1040 steel were peened at oil pressure of 50 MPa. Residual stresses induced on the oil jet peened specimen was in the order of -200 MPa. Standoff distance influenced the residual stress induced and also the erosion and surface roughness. Fully reversed cantilever bending tests conducted on the peened and unpeened conditions revealed the improved performance of the oil jet peened specimens.

Abstract: Two MCrAlY overlay coatings (NiCrAlYSi and NiCoCrAlYHf) were deposited on a DS superalloy of Ni-Al-Cr-Co-W-Mo-Ta-Hf system by vacuum arc deposition method. The two coatings show a good protection for the DS superalloy during the isothermal oxidation test at 1150 for 100 h. A continuous alumina-based scale was formed at the surfaces of the coated specimens after oxidation. Y2O3, NiO and SiO2 are also detectable in the oxide scale. For the Hf-bearing coating, some HfO2 particles exist at the interface between the coating and the substrate. But after the cyclic thermal oxidation at 1100°C, the Hf-free coating exhibited better oxidation resisitance than the Hf-bearing coating. The HfO2 blocks between the coat and the substrate may be the crack initiation which leads to the failure of the coating.

Abstract: A creep resistant Mg alloy MRI 230D was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. The laser surface treatment produced a fine dendritic microstructure and this treatment was beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarisation resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarisation resistance values of laser treated material were twice as high as that for the untreated material. This improved behaviour was due to the finer and more homogenous microstructure of the laser treated surface. The laser treatment also increased surface hardness two times and reduced the wear rate by 25% due to grain refinement and solid solution strengthening.

Abstract: The paper presents investigation results of tribological and cutting properties of the coatings deposited with the PVD and CVD techniques on cutting inserts made from the Al2O3 + TiC tool ceramics. Tests were carried out on the inserts made from ceramics, uncoated and PVD or CVD-coated with gradient, mono-, multilayer and multicomponent hard wear resistant coatings composed of TiN, TiCN, TiAlN, TiAlSiN and Al2O3 layers. Substrate hardness tests and micro hardness tests of the deposited coatings were made on the ultra-micro-hardness tester. It was demonstrated, basing on the technological cutting tests of grey cast iron (260 HB), that putting down onto the tool ceramics the thin anti-wear PVD and CVD coatings increases their abrasion wear resistance, which has a direct effect on extending tool life of the cutting edge.

Abstract: This study aims at correlating microstructure with hardness and corrosion resistance of surface alloyed materials fabricated with Fe-based metamorphic powders by an accelerated electron beam irradiation method. The surface alloyed materials contained 48 vol.% of hard Cr2B crystalline phases in the Cr0.19Fe0.7Ni0.11 matrix, and thus its hardness was 2.5 times greater than that of the steel substrate. The corrosion resistance of the surface alloyed materials was better than that of an STS304 stainless steel or coatings fabricated by high-velocity oxygen fuel spraying of Fe-based metamorphic powders because the Cr0.19Fe0.7Ni0.11 matrix of the surface alloyed layers and coating was selectively corroded, while Cr2B borides were retained inside pits. These findings suggested that the fabricated surface alloyed materials presented good application possibilities as excellent wear- and corrosion-resistant materials.

Abstract: A novel spray forming process was developed to produce large billets, wide plates or thick tubes with excellent microstructures and high cooling rates. Its uniqueness lies in a combination of the wide-range reciprocating movement and the swing scan of a gas atomizer, and the externally forced cooling of substrate during this spray deposition procedure. Its basic concept is that both good sticking and rapid solidification can be achieved if droplets with high liquid fractions impact a cold substrate, spread fully and then deposit on the surface. In order to control and optimize this new process, the thermal histories of droplets and deposits for spray forming of aluminum alloy billets were simulated with a set of new numerical models. Through shortening spray distance and raising melt superheat properly, the liquid fraction of droplets before deposition will increase and their spread on the deposit surface can improve for good sticking. Simulation results show that the optimal liquid fraction of droplets for deposition is about 0.2 higher than that in the conventional Osprey process. Its optimum spray distance is about 0.25m, which is nearly half as that in the Osprey process. In addition, this new process increases the mushy layer area and the specific surface area of heat extraction during deposition. Together with the forced cooling of substrate, it results in higher cooling rates. A high-quality large billet can be obtained by controlling the atomizer movement, the droplet liquid fraction and the deposit surface temperature properly in this new process.

Abstract: Development of complex ternary nitride coatings has attracted significant industrial interest in recent years. In deposition of complex ternary nitride coatings, the nitrogen deposition pressure plays an important role in structural evolution of the coatings leading to development of different mechanical properties. This paper summaries some successful analyses by the authors on the relationships amongst the deposition rate, grain size and hardness of the coatings against the nitrogen deposition pressure. It has been established that as the nitrogen pressure decreases, the deposition rate of the coatings increases and the grain size decreases. Hardness of the coatings increases due to the development of a refined and densified coating structure. Taking into account of the reaction kinetics at the targets, the interactions of the sputtered atoms occurred in their transfer to the substrate, the reaction kinetics at the substrate, the target material characteristics and the geometric arrangement of the sputter magnetron configuration, modelling to the relationships of deposition rate with nitrogen deposition pressure, grain size with deposition rate and hardness with grain size have been successfully established in this study. A limiting grain size of the coatings has also been identified in the grain refinement process.

Abstract: The paper summarizes new and published data and also deals with structure, applications, and properties of gasars – new porous materials based on gas-eutectic reaction which was establish in USSR in 1976-1980. The method consists of melting a material in an active gas atmosphere and solidifying under controlled conditions. The materials produced by this method, have a solid matrix and pores of varied geometric shapes, providing to gasars much higher strength, plasticity, thermal and electrical conductivities as compared with those of other porous materials. For USA gasars can be recommended for there are many applications for civil and military industries.

Abstract: Formation of a protective scale such as chromia or alumina is a prerequisite for an excellent oxidation performance of high-temperature materials. Based on this principle, a novel concept of designing chromia- or alumina-forming nanocomposites by means of nanocomposite electrodeposition of Ni with Cr or/and Al nanoparticles were proposed. A brief review on the high temperature oxidation performance of such novel electrodeposited nanocomposites like Ni-Cr, Ni-Al and Ni-Cr-Al is presented.