From the appearance of high velocity oxygen fuel (HVOF) thermal spray system in the 1980s, WC cermet coatings have been used as anti-wear coatings in many industrial manufacturing applications. Recently, WC cermet spray materials were applied using new thermal spray methods such as warm spray and cold spray, which are still in the research phase. In HVOF spraying, WC-Co and WC-Ni powders are regularly used as coating materials. On the other hand, using cold spray, WC-Fe alloy series can be deposited as dense and thick coatings, better than WC-Co. In this study, WC-Fe alloy powders were sprayed by cold spray to investigate the influence of binder metal on the coating properties and compared with those of HVOF WC-CoCr coatings. It was observed that the lower metal ratio and FeCrNi chemical composition exhibited improved results.

Iron based coatings have recently gained much attention as they have favorable mechanical, frictional, and corrosion properties. The coatings possessed a high content of iron borides are particularly valuable for satisfying engineering needs. Boron and iron form two major boride phases, FeB and Fe2B, with different mechanical and thermal properties. Orthorhombic boride FeB is considered to be viable candidate to enhance the surface hardness and wear resistance of components, since it has high hardness. Producing of such coating by cold spray method is considered to be an alternative for boronizing method which is a conventional thermochemical surface hardening process. In this work, the crushed ferroboron (FeB) powders of Fe-17.9B-0.4C-1.6Si-0.3Al (wt. %) were deposited onto low carbon steel substrate by cold spraying. However, low and high pressure cold spraying allowed depositing very thin and single layer on the substrate, due to the intrinsic brittleness of the powder. Therefore, several contents of Al, Ni and Fe metallic powder and their combinations were added to FeB powder to obtain thick coating via cold spray processes. Post heat treated coatings at a temperature of 700 °C resulted in increase of the hardness, possibly the formation of hard phases such as intermetallic compound.

Cold spraying is well known as an attractive coating process that prevents degradation of materials, such as oxidation, decomposition and undesirable reaction due to overheating. High velocity oxy-fuel (HVOF) spraying has been recognized as the most popular technique in particular to produce tungsten carbide (WC) cermet coatings in industrial fields. However, the degradation of WC cermet occurs using HVOF that is quite different from sintered hard metal, which can be defined as an ideal material. Thus, low temperature processes, such as cold and warm spraying, are actively researched to achieve WC cermet coatings, similar to sintered one. In this study, influence of gas conditions in cold spraying on WC cermet coatings has been investigated, where WC-Co, WC-CrC-Ni and WC-Fe alloy are selected. The study reveals that nitrogen gas is effective to form thick, dense and hard coatings of WC-CrC-Ni and WC-Fe alloy compared to helium gas. This suggests optimal temperature and velocity of powder jet has been formed when using nitrogen gas.

From the appearance of high velocity oxygen fuel (HVOF) thermal spray system on 1980s, WC cermet coatings have been used as an anti-wear coating in many industrial manufacturing. Recently, WC cermet spray materials were applied to new thermal spray methods such as warm spray and cold spray under research phase. In HVOF spraying, WC-Co or WC-Ni series powder are used as standard contents. Ni and Co are binder metals for WC because of good wettability and suitable melting point. On the other hand, warm spray and cold spray are lower temperature process than HVOF. It is considered that any other factors of metal material such as hardness, toughness and crystal phase should be investigated in warm spray and cold spray. In this study, WC with Co or Fe alloy powders were sprayed by cold spray and HVOF to investigate the influence of binder metal for spray efficiency and coating property. It is cleared that Ni and Fe were superior to Co in spray efficiency and coating property in cold spray. The detail of above reason was under investigation, however, plastic deformability of binder metal is expected to be an important factor for WC cermet cold spraying.

Cold spray deposition of polycarbonate on the various substrates has been investigated. The polycarbonate particles are sieved and accelerated at elevated temperature in air through a DeLaval type nozzle, and are deposited on the metallic and ceramic substrates. The influences of the particle size, the gas temperature, the thermal conductivity and surface roughness of substrate on the deposition process are studied. As a result, the continuous deposits are formed on the metallic substrate. The powder sieved below 300 μm shows better deposition efficiency. Thin film of melted polycarbonate has been formed on the surface of substrate to act as a bonding layer, and its crystalline structure is changed to be amorphous, which is the more stable state for the polycarbonate. The coating seems to be better when the thermal conductivity of metallic substrate is low. For the ceramic substrates, there is no deposition whatever was the thermal conductivity.

Thermally grown oxide (TGO) grows at the top / bond coating interface of the thermal barrier coating (TBC) in service. It is supposed that the failures of the TBC occur due to thermal stress and the decrease of adhesive strength caused by the TGO growth. Recently, large local stress has been found to change both the diffusion constant of oxygen through an existing oxide and the rate of chemical reaction at the oxide / oxidized material interface. Since high thermal stress occurs in the TBC, the volume expansion of the newly grown oxide, and centrifugal force, the growth rate of the TGO may change depending on not only temperature but also the stress. The aim of this study is to make clear the influence of stress on the growth rate of the TGO quantitatively. As a result, the thickness of the TGO clearly increases with increase of the amplitude of the applied stress and temperature. The increase rate of the TGO thickness is approximately 23% when the applied stress is increased from 0 to 205 MPa at 900 °C, and approximately 29 % when the stress is increased from 0 to 150 MPa at 950 °C.

The effect of particle size range on oxidation behavior was investigated according to exposure time in isothermal oxidation condition. Emphasis was placed upon oxygen content, porosity, and oxide scale formation. Commercially available CoNi- and CoCrAlY powders of several different particle size ranges were vacuum-plasma sprayed on a nickel alloy substrate. The results show that the isothermal degradation of coatings is considerably influenced by the particle size distribution. It can be clearly observed that a remarkable increase in the oxygen content in the as-sprayed coating occurred with a decrease in the mean particle size. But after thermal exposure, the difference of the oxygen contents between the smaller and larger particle coatings is decreased. The distribution of particle size plays the important role of porosity than only the mean particle size. The powder which has the widest range and sample variance leads to make good porosity inside coatings during the deposition process.