According to a recent survey, the thermal spray market in Japan is approximately 100 billion yen. The market consists of three sectors, coating services (45 billion yen), in-house production (45 billion yen), and sales of thermal spray equipment and consumables (10 billion yen). This paper provides an overview of the coating services market broken out by application segments. It also discusses future trends.

This study compares the performance of an ordinary plasma spray gun with that of recently developed gun designed for spraying feedstock suspensions on inner diameter surfaces. To evaluate the guns, yttrium oxide was deposited on 304 stainless steel while varying supply pressure, spraying distance, and gun traverse speed. Different methods of delivering suspension spray material to the gun were also investigated. Although Y 2 O 3 inner-diameter coatings were successfully formed, hardness and cross-sectional porosity need improvement. Based on the findings, it may be necessary to increase substrate temperature, readjust spraying parameters, and optimize feedstock materials.

A three-dimensional model of free-surface flows with heat transfer, including solidification, was used to model the build-up of a coating layer in a thermal spray process. The impact of several nickel particles on a stainless steel plate in different scenarios was considered. Particles diameter ranged from 40 to 80 µm and their impact velocity ranged from 40 to 80 m/s. Particles were initially super-heated; their temperature ranged from 1600 to 2000°C. Fast growth of solidification was found to be one cause of particle splashing in thermal spray coatings. Different splat morphologies obtained from the numerical model were comparable with those obtained from the experiments. Simulation of the sequential impact of two nickel particles showed side-flow jetting and particle splashing observed in experiments. The numerical model proved to be capable of simulating different impact scenarios that occur in a thermal spray; this was demonstrated by simulating nine consecutive particles during their impact on the substrate. Several characteristics of a coating layer build-up such as particle splashing and formation of small satellite droplets and rings around the splat could be seen in the numerical results. Particle splashing is one possible cause of porosity formation in thermal spray coatings.

A numerical model is developed to study the effects of the contact resistance, droplet impacting droplet temperature, and substrate temperature on the droplet solidification rate and temperature of the droplet under the condition when the substrate can melt and re-solidify. Two-dimensional simulations show that the interface velocity is small in the area of poor contact with an irregular solidification interface shape. During the impact of Molybdenum on a steel substrate, Mo solidifies while the steel substrate melts.

The microstructure of thermally sprayed ceramic coatings is characterized by the existence of various pores and microcracks. The porous microstructure makes coating desirable for thermal insulation, but this unique microstructural feature also gives rise to anelastic response under tension and compression loads. Detail investigations of curvature measurements of ceramic coated substrate indicate the coatings to exhibit anelastic behavior composed of nonlinear and hysteresis characteristics. In this paper, the mechanisms of such behaviors were studied from curvature-temperature measurements and finite element analysis through modeling the microstructure of yttria stabilized zirconia (YSZ) coating. Computational models contain numerous randomly distributed pores and microcracks with various sizes, aspect ratios, locations and orientations. The effects of such attributes of pores and microcracks on coating anelastic behavior were studied by simulations of curvature change during thermal cycles.

Although it is widely used prior to the spraying stage to eliminate the surface contaminants and to improve the splat contact with substrate, a preheating operation should be precisely controlled to keep the metallic substrate away from an excessive oxidation. In most cases, the oxide scale can deteriorate the interface adhesion. A nanosecond pulsed laser treatment is a powerful tool to remove the surface oxide layers. It is also the fundamental principle of the PROTAL process that combines a laser surface preparation and the thermal spraying process. In this paper, a nanosecond pulsed laser was used to remove the thermally induced oxide while keeping the advantage of preheating. It is shown that the particle splashing was suppressed and the deposit adhesion was significantly improved resulting from the laser cleaning effects. The mechanism of oxide removal is also investigated.

A laminated coating, using the thermal spray and other processes, protects a metal substrate in a highly corrosive and highly erosive environment. The combination of noble metals and ceramics offer highly bonded, pore free coatings for demanding industrial applications. This paper offers case examples to exemplify some success stories.

Alloys with high amount of chromium are used for protection of power plant boilers against high temperature corrosion, especially heat exchanger surfaces, such as superheaters. Thermal spray coatings can be applied to low alloyed steel in order to enhance the lifetime of the cheap substrate material. In this study NiCr (50Ni49Cr1Fe) powder was sprayed with DJ Hybrid and CJS HVOF techniques, and the spraying process was optimized with process map methodology, including SprayWatch for measuring the temperature and velocity of the spray stream, and ICP (In-situ coating property measurement) for measuring the stress state. Different spraying parameters were applied in order to attain the best suitable coating characteristics for high temperature applications, such as high density and low oxidation stage. The coatings were analyzed e.g. by microscopic means and mechanical testing. The use of process optimization, process-structure- properties methodology, and following coating characteristics are presented.

Industrial robots are widely used in thermal spraying operations in order to assure process accuracy and reproducibility, as well as to increase worker´s safety. Robot path planning for the coating of free form geometries keeping constant the kinematic parameters can lead to complex robot paths and in some cases to unreachable robot configurations. An external rotational axis in coordinated movement with the robot can be used to manipulate complex shaped components during the spraying operation and to simplify the robot movement. Moreover, the robot kinematics, which define the spraying path, spray angle, spray distance and spray velocity, are key parameters to control the heat and mass transfer to the substrate during the coating deposition, and they influence therefore the development of residual stresses. The present work describes the off-line planning of robot trajectories for thermal spraying operations using an external rotational axis taking into consideration the heat transfer to the component.

Coating adhesion has a major effect on the performance of thermal spray coatings and is in large part determined by the chemical, physical, and mechanical properties of the substrate surface. This paper reviews some of the work that has been done to optimize the surface of different materials for thermal spraying. It covers a number of substrate preparation methods, including grit blasting, laser ablation, laser surface texturing, and dry ice blasting, and explains how different process variables affect surface roughness and bond strength.

The chemical and physical properties of thermally sprayed coatings depend in part on surface characteristics, such as texture and roughness, that can be difficult to quantify. This paper demonstrates the use of fractal analysis for quantifying surface roughness based on images obtained from cross-sectioned samples. An isolated profile of an Al 2 O 3 -TiO 2 coating on an aluminum substrate is used as the test case. The first part of the paper covers the basic concept and the procedures used. The second part presents practical examples and precautions that must be observed to obtain accurate and reproducible results. Paper includes a German-language abstract.

In order to be suitable for food processing application, besides having the usual characteristics as high adhesion, high cohesion, high compactness and maximum hardness and wear resistance, the thermal spray coating should not release foreign substances, as prescribed by the international standards. This paper defines a very strict procedure according to valid EC and FDA standards in order to test the compatibility of the coating with the food. It discusses the applicability of this test method, in which a contact is created between a food-simulating solvent and the thermally sprayed coating to be analyzed. The inert nature of the drawn migration cell, the adopted time-temperature conditions and the characterization of the coating before and after the migration test are discussed. Paper includes a German-language abstract.