A conventional GTV K2 kerosene fuel HVOF spraying system has been modified with the aim to expand the applicable range of process conditions in order to also cover the presently existing gap between conventional HVOF and cold gas spraying. Different measures have been applied in order to reduce heat transfer and increase momentum transfer to spray particles. Increase of momentum transfer results both in reduced particle temperature as a consequence of reduced dwell time in the hot flame area and high particle velocities. In detail expansion nozzles with conical shape that provide improved expansion of combustion gases, combustion chambers with reduced critical diameters that provide increased combustion chamber pressures and the injection of coolant media water and / or nitrogen into the combustion chamber are applied. The effect of modified spraying conditions on the process characteristics and coating properties have been studied for a variety of materials. Besides copper, titanium alloys and MCrAlYs also WC/Co(Cr) and Cr 3 C 2 /Ni20Cr have been sprayed and respective coatings have been analysed concerning their microstructure, gas content, microhardness and wear resistance. In particular a moderate increase of oxygen content in dense titanium alloy coatings compared to the powder feedstock from 0.41 to 0.59 wt.-% proves the high potential of the undertaken measures to expand the application field of HVOF spraying. Thereby existing systems only need relatively small modifications to achieve this expansion.

High velocity oxygen fuel (HVOF) sprayed WC coatings have been found wide applications as substitutions of chrome plating in the airplane industry. The wear of HVOF WC coated landing gear against Al-Ni-Bronze alloy leads to the wear or spalling of WC particles, and finally results in the degradation of sealing property for landing gear system. WC-Co coating with nano-size WC particles is expected to exhibit good wear resistance, and the weight loss of Al-Ni-Bronze alloy can also be reduced, which is beneficial to the sealing property of landing gear system. Three types of coatings are prepared using agglomerated, sintered nano-scaled WC-12Co, conventional WC- 12Co and WC-10Co4C powders, respectively. The wear behaviors of the as-sprayed coatings against Al-Ni- Bronze alloy are investigated at the different loads and linear velocities. The study is expected to provide the basis for the application of HVO/AF nano-scaled WC coating in landing gear.

Thick pure tungsten coatings were deposited on molybdenum and copper substrate by vacuum plasma spray for different purpose. The microstructures of tungsten coatings were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of preheating temperature of the substrate and introduction of gradient bond layer on the adhesive strength of the coating was investigated. High heat load was tested by high power electron beam bombardment only for tungsten coating on copper. All the results show that the properties of tungsten coating were strongly influenced by different processes, and the density of the coatings is close to 95% of theory density.

In order to improve the wear resistance and service life of the copper, the composite coating consisting of a Ni-base self-fluxing alloy (NiCrWB+50%Al 2 O 3 ) and WC (WC-12%Co) alloy were sprayed on a copper substrate using High Velocity Air Fuel(HVAF). The coating could meet the operating requirements including high hardness, good wear resistance and low cost. The Ni-base composite coating was analyzed by means of optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The results indicated that the structure of coating was composed of melted particles and partly unmelted round particles of Ni-base alloy, and WC particle. Only a small proportion of the Al 2 O 3 particles were retained in the coating. The phases in the coating consisted of γ-Ni, WC and a little Ni 3 B. Amorphous structures appeared and some Al 2 O 3 phase existed. The adhesion strength between coating and copper substrate was more than 50MPa. Wear results showed that the Ni-base composite coating exhibited better wear resistance than the coating with no WC particles.