In this study, MCrAlY-Al 2 O 3 composite powders were produced by ball milling and deposited by plasma, HVOF, and cold spraying. The results show that Al 2 O 3 fractions can be well controlled using composite powder due to non-preferential impact debonding of the matrix and Al 2 O 3 . The microstructure of spray powders is well retained in HVOF and cold-sprayed coatings due to the unmelted or partially molten condition of the spray particles. In the case of plasma-sprayed coatings, however, most Al 2 O 3 particles segregate at lamellar interfaces, forming a continuous oxide scale on the splat. The cold-spray coatings exhibit the highest hardness due to the work hardening effect of kinetic deposition.

In this study, WC-CoWC coatings were produced by HVOF spraying using bimodal-structured WC-Co powder with both micro- and nano-sized WC particles. Due to the melting characteristics of the powder during spraying, the microsized particles are retained in the deposit, but the nanosized particles dissolve into the Co matrix, forming a Co-W-C ternary phase. Compared to coatings sprayed from conventional WC-CoWC powder, the bimodal coatings are more resistant to corrosion and wear and are comparable in microhardness.

Sub-micro-structured titanium nitrides (TiN) coatings on Al 2 O 3 substrates were fabricated by vacuum cold spray (VCS) process using ceramic powders, which were ball-milled at room temperature. The microstructure features and crystal structures of the VCS TiN coatings were analyzed by scanning electron microscopy and X-ray diffraction. The adhesion between the coating and the substrate was evaluated with a scratch tester. The sheet resistance of the VCS TiN coatings was measured by using a four-point probe method. The effects of nozzle traverse speed on the microstructure, adhesion to substrate and electrical properties of the coatings were investigated. It was found that the adhesion improves greatly with the nozzle traverse speed increasing from 5 to 15mm/s, and the electrical resistivity levels of the coatings is decreased significantly. The resistivity of sub-micron-structured TiN coatings is substantially lower than those of nano-structured ones fabricated by the same VCS process. And a minimum resistivity of 1.16×10 -4 Ω·m is achieved.

Three types of cobalt-based cermet coatings were prepared by high velocity oxy-fuel (HVOF) spraying using cobalt- clad TiC-50Co, SiC-50 Co and WC-18Co powders. The microstructure of three coatings was characterized using a scanning electron microscope (SEM). The adhesive strength of the coatings was tested according ASTM C633-79 standard. The hardness of three coatings was measured using a HV-5 Vickers hardness tester. The abrasive wear performance of the coatings was examined by a dry-sand rubber wheel tester according to ASTM G65-61 standard. The results show that the density, thermo physical properties and volume fractions of the solid carbide phases in the spray particle have a significant influence on the adhesive strength of the coatings. The hardness of WC-18Co coating is higher than that of TiC-50Co and SiC-50 coatings and is much lower than WC-17Co coating deposited with sintered-crushed powders. Moreover, the abrasive wear volume loss of the WC-18Co coating is about 60 times higher than that of the WC-12Co coating sprayed by sintered-crushed powder, and greatly lower than that of TiC-50Co and SiC-50 coatings. The wear mechanisms of three coatings are discussed.

Composite powder in sub-micrometer size was prepared using a primary nano-TiO 2 powder and polyethylene glycol (PEG). The nano-TiO 2 coating was deposited through vacuum cold spray using both the composite powder and primary nano- TiO 2 powder. The influence of annealing treatment on coating adhesion and photocatalytic activity was investigated. The coating adhesion was estimated through erosion test by water jet. The photocatalytic performance of the TiO 2 coatings was evaluated through photodegradation of phenol in water. The results showed that annealing of the coating at a temperature from 450 to 500 °C yielded both higher activity and better adhesion. The adhesion of the coating deposited using the composite powder was better than that using primary nano- TiO 2 powder. It was found that TiO 2 coating resulting from the composite powder presented much higher activity than that deposited with primary nano-particles. The better activity is attributed to the existence of large pores resulting from the stacking of composite powder, which benefits the reactants transportation through the porous coating.

Silver ion was added to liquid feedstock to deposit Ag+ doping nanostructured TiO 2 photocatalytic coatings through liquid flame spraying. The coating microstructure was characterized by X-ray diffraction (XRD). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. XRD analysis showed that the phase structure of coatings was not significantly influenced by the silver ion doping. However, a shift was found for XRD peaks of anatase TiO 2 . The photocatalytic activity of the TiO 2 coatings was increased and then decreased with the increase of dopant concentration. The photocatalytic activity of doping coatings was higher than that of pure TiO 2 coating, in spite of dopant concentration. The enhancement of photocatalytic performance of doping coatings is attributed to co-doping of Ag+ ion and metallic Ag.

Nano-structured WC-Co coating was deposited by cold spraying using a nanostructured WC-12Co powder. The critical velocity for the particle to deposit was measured. The coating microstructure was characterized by X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. The coating hardness was tested by using a Vickers hardness tester. The deposition behavior of single WC-Co particles was examined. WC particle size was also measured for comparison of deposit properties to that of sintered bulk. The result shows that the nanostructured WC-Co coating can be successfully deposited by cold spraying using nanostructured powders. The coating exhibited a dense microstructure with full retention of the original nanostructure in the powder to the coating. The test of microhardness of the coating yielded a value of over 1820 Hv0.3, which is comparable to that of sintered nanostructured WC-Co. The deposition behavior of WC-Co powders as superhard cermet materials in cold spraying and powder structure effects are discussed.

Copper ion was added in liquid feedstock to deposit ion doping TiO 2 photocatalytic coatings through liquid flame spraying. The coating microstructure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. XRD analysis shows that the crystalline structure of coatings is not significantly influenced by the metal ion doping. The photocatalytic activity of the TiO 2 coatings is enhanced by the copper ion doping. It is found that a high concentration of ion doping decreases the activity. XPS analysis shows that the adsorbed oxygen concentration is increased with the increase of Cu 2+ dopant concentration and decreases with the further increase of dopant concentration. The enhancement of photocatalytic activity can be attributed to the adsorption ability of oxygen and other reactants on the surface of doping TiO 2 coatings.

Impacting of a molten droplet with melting point much higher than substrate results in melting of substrate around the impact area. The melting of the substrate surface to certain depth alters the flow direction of droplet fluid. The significant change of fluid flow direction leads to detaching of fluid from contact with the substrate. Consequently, splashing occurs during droplet spreading process. In the present study, Mo splats were formed on stainless steel substrate under different plasma spraying conditions. For comparison, Mo splats were also deposited on Mo surface. The substrate surface was polished prior to deposition. The powders used have a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was the split type presenting a small split structure on stainless steel substrate surface. With Mo particles, the preheating of steel substrate has no effect on splat morphology. On the other hand, disk-like type Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrate and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface-melting- induced splashing model was proposed to explain the formation mechanism of Mo splat on steel surface.

For high velocity oxy-fuel (HVOF) sprayed Cr 3 C 2 -NiCr coating, an experimental method to evaluate the carbon and carbide loss of the particles in the stages of both in-flight and impacting on a substrate was proposed. The carbon loss in these two stages was determined by comparing the carbon content in the starting powder, the collected powder sprayed into water and the coating deposited on the substrate. The carbide loss caused by carbide rebounding, dissolving into NiCr matrix and oxidizing in-flight can be determined by evaluating quantitatively the carbide content of the collected powder and the deposited coating. According to the experimental results, it was revealed that the rebounding off of large carbides during particle impact on the substrate is mainly responsible for the carbon loss. The carbide rebounding off and dissolving are two main reasons for the carbide reduction in the coating. The carbon and carbide loss caused by oxidizing during the in-flight of particles is very limited.

A typical feature of cold spray process is that a deposit can be formed without change of the original structure and compositions of spray materials. Only particles which reach the velocity higher than the critical velocity can be deposited on a substrate in cold spraying. When the spray particle impacts on the substrate at an off-normal angle, the normal component of particle impact velocity will change with the approaching angle of spray particle to substrate. In the present study, copper and titanium powders are used to deposit coating using cold spray process at different impact angles with regard to substrate. The deposition characteristics of spray materials are examined. The results show that the impact angle has a significant influence on the deposition characteristics. The relative deposition efficiency changes with the spray angle. It has been found that there is a critical impact angle at certain particle conditions below which no deposition occurs. The relation between spray angle and relative deposition efficiency can be divided into three spray angle ranges: maximum deposition angle range, transient angle range and no deposition angle range. In the transient angle range, the relative deposition efficiency increases with an increase in spray angle from zero at the critical spray angle to 100%. The transient angle range depends on the particle velocity distribution. A model is proposed to explain the relation between the spray angle and the relative deposition efficiency.

HVOF cermet coatings have been widely used to improve the wear resistance. In the present study, an abrasive wear model is proposed according to the examination of wear mechanism. The relation between the relative abrasive wear and the size and content of carbide particles in cermet coating is established. HVOF WC-Co and Cr 3 C 2 -25NiCr coatings are deposited to correlate experimentally the relation between the carbide particle size and abrasive wear. The microstructure of coatings is examined using scanning electron microscopy (SEM). The carbide particle size in the coatings is measured according to SEM images of the coating. It has been found that experimental results correlate with the theoretical model well. The abrasive wear of HVOF carbide cermet coating is determined by carbide size and content. The relative abrasive wear resistance will be increased with an increase in carbide content and the decrease of square root of relative carbide particle size.

This paper examines the bonding characteristics of HVOF sprayed layers. In order to determine the dominant adhesion mechanism, the coatings are applied to substrates of varying surface roughness with powder particles that are completely melted, in one case, and only partially melted, with solid and liquid phases, in another. The spray materials are NiCrBSi and WC-Co. The results of adhesion tests showed that the adhesion of the NiCrBSi layer on the roughened substrate was approximately 40 MPa, while there was no adhesion on the polished substrate. In contrast, the adhesion strength of the WC-Co layer on the polished substrate was between 20 and 40 MPa, and on the roughened substrate, it exceeded that of the binder materials. Paper includes a German-language abstract.

NiCrBSi and Ni-50Cr coatings are deposited using High Velocity Oxygen-Fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of melting state of spray particles on the adhesive strength of the coating. The adhesive strength of coating is estimated according to ASTM C633-79. The melting state of spray droplet is examined from the coating microstructure. It is found that the melting state of spray particles has significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of spray particle does not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of partially melted large particle contributes to the substantial improvement of adhesive strength of HVOF coating. The subsequent coating presents a dense microstructure and yields an adhesive strength of over 76 MPa, which is doubled compared to the coating deposited with completely molten particles.

The effects of powder types and HVOF spray systems used to produce Cr 3 C 2 -NiCr coating on the relationships between spray parameters and wear performance were investigated based on the effect of fuel gas conditions on abrasive wear and erosion wear. The relationships between spray parameters and wear properties were obtained by orthogonal regression experimental design method. Four types of powders and two HVOF spray systems were used. It is found that with the increase in fuel gas flow or pressure the abrasive wear and erosion of Cr 3 C 2 -NiCr coatings change following a concave curve. The Cr 3 C 2 -NiCr coating with the best wear performance will be deposited under intermediate fuel gas condition. It is experimentally confirmed that by different types of powders and HVOF systems applicable to HVOF spraying of Cr 3 C 2 -NiCr coating, although the optimized fuel gas conditions to deposit coating with the best wear performance will be influenced by types of starting powders.