In recent studies, titania has been added to hydroxyapatite (HA) coatings to impart photocatalytic properties. The benefits of such additions are maximized when titania is in nanocrystalline anatase form. In this study, nano-titania was synthesized in-flight from a liquid precursor consisting of ethanol and titanium isopropoxide. The precursor and HA powder were fed into a plasma gun, forming nano-titania particles that embedded in the HA. Coatings of pure titania and titania-embedded HA were deposited under different spray conditions on titanium coupons and then characterized via XRD and SEM analysis. The titania coatings contained ultrafine anatase and rutile particles with anatase being favored by more power input and rapid quenching. The composite coatings contained dispersed ultrafine titania particles in a matrix consisting primarily of HA with trace amounts of calcium phosphate and amorphous phases. The effect of spraying parameters on phase and microstructure evolution is discussed as well.

This work shows that hydroxyapatite (HA) can be cold sprayed simultaneously with titanium to form thick biocompatible coatings without compromising the phase constituents of the bioceramic material. XRD analysis indicates that the phase composition of the HA in the deposit is identical to that of the powder. The work also shows that very dense Ti and Ti-HA composite coatings can be produced using sponge Ti powders and nitrogen process gas. The adhesion strength of the cold-sprayed Ti-HA exceeded the reported values of comparable plasma-sprayed coatings.

This study evaluates the potential of functionally graded composite coatings fabricated with a novel spraying process. The hybrid process embeds varying amounts of ceramic particles in a metal matrix by combining HVOF and wire arc spray streams. In this case, ceramic nanoparticles (SiO 2 , Cr 2 O 3 , Al 2 O 3 ) from liquid precursors are embedded in a matrix synthesized from NiCr wire feedstock. The resulting coatings are evaluated based on high-temperature performance and the composition and dispersion of particles in the NiCr matrix. Uniform size distribution and dispersion of nanoparticles is shown to correlate not only with denser coatings, but also improved corrosion, wear, and oxidation resistance.

This study investigates arc instability phenomena associated with hollow-cathode plasma torches under low pressure. Stable and unstable regimes are evaluated based on Fourier analysis of gun voltage. For comparison, arc fluctuations with a solid-cathode torch are studied as well. Different internal hardware configurations are used during spraying to identify and assess important factors in the design of hollow and solid cathode guns. This work also characterizes a number of parameters that would exploit plume elongation and accelerated gas velocities under low-pressure conditions.

Arc instability in dc plasma guns has a detrimental effect on coating quality and electrode life. This paper investigates the impact of a single-cathode plasma column design on process stability, versatility, and component life. The design increases arc length and minimizes restrike, enabling high voltage, low current operation as well as the use of ternary gas to increase enthalpy. Arc behavior is assessed by monitoring fluctuations in electrical signals and correlated with particle temperature and velocity. The takeover-steady mode is shown to be the typical operating mode of the plasma jet for this design, resulting in steady particle characteristics over long periods of maintenance-free operation.

The hybrid spray process that combines arc-spray with HVOF/plasma jet, has recently demonstrated its effectiveness in the deposition of functionally gradient coatings. This approach aims at exploiting the attributes of both the arc-spray technique combined with those of the HVOF/APS technique. This paper presents high speed visualization and plume characterization of an Arc/HVOF hybrid spray gun as well as a twin-wire arc spray gun. The physics of atomization in hybrid spray process is examined using high-speed camera. A DPV-2000 particle diagnostics sensor is used to measure particle velocity, temperature, size and distribution. The influence of feed material, arc-spray parameters, and the jet parameters on the particle characteristics is presented. The differences in the in-flight characteristics between the hybrid and the twin wire arc process have been discussed aided by the observed atomization phenomena with the high speed camera.

A simulated environment erosion test platform consisting of a control chamber, a modified HVOF gun, and a programmable logic control module is presented. This robust equipment can vary angles of impingement from 0-90 degrees, erodent velocities up to 300 m/s, and temperatures exceeding 2000°C. The erodent velocity, flow rates and temperatures are directly measured employing a high speed camera, and a DPV/CPS 2000 particle flight diagnostic sensor in contrast to estimates typically reported in the literature. It is demonstrated that, although the gas jet type erosion test apparatuses are frequently used in laboratories, quantitative parameter characterization is essential for setting up a test protocol. Both room and high temperature tests have been performed on this test bed. It is believed that the quantitative characterization of test parameters would greatly assist the designer in choosing and developing the appropriate erosion resistant coatings for their application.

Functionally graded material (FGM) designs aim to combat challenges posed by harsh and conflicting operational requirements. There is a great demand for manufacturing technologies that can fabricate net-shape components incorporating the FGM design concepts. The focus of this paper is on the development of net-shape components incorporating functionally gradient material designs. This novel manufacturing scheme combines the use of re-useable mandrels to generate complex shapes with a unique thermal spray process called “hybrid spray” that can deposit functionally designed materials. This generic hybrid spray process combines arc spraying with either high velocity oxy-fuel (HVOF) or plasma spraying (APS). The resulting benefits are; ability to achieve continuous composition variation, high deposition rates combined with the high coating densities. The multi-material component designs aim to provide cost as well as performance advantages. The geometric resolution and dimensional accuracy of these functional components are presented. This paper also reports results on material compatibility and process parameter development tests. Functional properties such as high temperature resistance and thermal fatigue performance are also reported.

This paper presents process development efforts aimed at exploiting the benefits of both the electric arc-spray technique and the High Velocity Oxygen Fuel (HVOF) technique. This so-called hybrid process offers the benefits of using wire stock and high deposition rates of an electric arc-spray gun combined with the higher coating densities obtained using a HVOF gun. Various configurations starting from a single wire arc to 4-wire arc in combination with HVOF have been investigated. Experiments with single material wires to multi-material wire combinations have also been investigated. The feed rates of individual electric arc wire pairs and HVOF powder/wire are controlled independently allowing the creation of functional gradient materials (FGM). This gun has been used to create alloys as well as patterned deposits of different materials. The ability to create in-situ alloys and patterned deposits with excellent quality has been demonstrated.

The ability to respond timely and cost effectively is bringing rapid manufacturing techniques into the mainstream production/repair arena. The Army can benefit from such technologies to avoid long lead times and high costs of procuring and inventorying replacement parts. But, a technology that offers versatility in terms of product size, cost and the ability to fabricate production components rapidly is yet to be available. This paper describes thermal spray technologies as applied to rapid manufacturing, part refurbishing and reengineering. A hybrid thermal spray system that can deposit a variety of materials with high throughput and density has been developed. Several generic materials that can meet the functional requirements of many parts and at the same time can be easily deposited by a thermal spray technique are discussed. An expert system software assisting the operator from CAD information, reverse engineering to part refurbishing is presented. Examples on the use of multiple schemes to remanufacture a part are also discussed. These schemes offer new avenues to fabricate replacement parts and/or salvage damaged parts and can benefit the Army considerably.

Thermopile devices are useful in many applications such as thermoelectric generators, temperature sensors as well as heat sinks. This paper presents a rapid fabrication scheme for thermopiles by thermal spray technique. The scheme is based on direct deposition of two over lapping chromel/constantan layer separated by sprayed on copper connector on a tube. The insulating layer on the tube is obtained by curing a sprayed on sol-gel ceramic by the spray gun itself. Serially connected thermocouple junctions are created by machining a spiral track on the tube. Design and manufacturability of an exhaust waste heat recovery module has been discussed. The electrical and thermal characteristics of the thermopile module have been presented. These thermopiles are designed for high temperature and harsh environments.

Functionally graded coatings with continuous changes of microstructures and properties across the material are expected to have low residual and thermal stresses and improved bonding strength between base materials and the ceramic coatings. This paper presents a technique to produce high-performance graded coatings in which the mix of components in the coating changes continuously from the base materials out. A hybrid HVOF-Arc spray gun has been employed to create such coatings. The metallic matrix material is utilized in the form of wires that are fused by arcing process. A high velocity combustion jet carrying the ceramic particles atomizes the molten material and mixes the ceramic particles with the matrix material. The feed rate of the matrix material and the reinforcing material are controlled together giving a systematic variation of the reinforcement phase. Two material systems; WC-16%Cr 3 C 2 -Ni-5%Al and B 4 C-Al(5%Si) have been investigated. The in-flight particle characteristics of the process have been characterized. The resulting microstructures and process capabilities are discussed.

The high-velocity oxy-fuel (HVOF) gun represented a major development towards forming dense coatings. But, compared to the electric arc-spray process, the HVOF process is more difficult to apply and in general more costly. Therefore, some process development efforts have aimed at exploiting the attributes of both the electric arc-spray technique combined with those of the HVOF technique. Specifically, this hybrid process utilizes the electric arc spray benefits of using wire stock and high deposition rates combined with the higher coating densities obtained using HVOF. This paper presents an in-depth, in-flight particle characterization of a hybrid spray gun. A DPV-2000 particle diagnostics sensor was used to measure particle velocity, temperature, size and distribution. The influence of feed material, electric arc-spray parameters, and HVOF parameters on the particle characteristics is presented. It is observed that the velocity of the particles in the hybrid mode are slightly lower than the velocity of particles by HVOF alone but significantly higher than typical electric arc-spray velocities. In addition, the particle temperature in the hybrid mode isn’t significantly different than those by HVOF only. The particles produced by the hybrid gun tend to be more uniform and smaller compared to traditional electric arc-spray coatings.

A two-color imaging pyrometer system is employed to examine twin wire arc spray gun used in Spray Tooling Process. Key aspects analyzed in this study include particle temperature, particle velocity, and particle distribution. The influence of process parameters such as gas flow rate, type of gas (N 2 , and air), voltage and current, as well as, spray cap design has been studied. Influence of a super atomizer called the arc jet has also been investigated. The measurements were taken at 80 mm, 160 mm and 200 mm from the gun tip. Finally, the influence of the material on the spray characteristics has been examined. The material list includes 0.8% carbon steel, 0.8% Carbon steel with B, stainless steel, Molybdenum, Ni, Ni-Al and Copper. The type of gas used for atomization significantly influences the particle temperature and velocity. Lowest temperature is observed with nitrogen gas and the highest temperature with the arc jet. Arc jet also produced particles with significantly higher velocity. The flow rate did influence the particle velocity where as temperature wasn’t affected noticeably. The wire material has considerable impact on the particle temperature and velocity. Lower melting point alloys showed higher temperature whereas the velocity has complex dependence on the density and viscosity of the material. Cap type affects the temperature and velocity of the particle. As cap opening increases the temperature and velocity decrease.