Current developments in different industrial sectors show an increasing demand on thermally sprayed internal diameter (ID) coatings. But up to now, the research focus is mainly on conventional processes such as arc spraying and plasma transferred wire arc spraying (PTWA), especially for cylinder liner surfaces. However, efficient HVOF and APS torches are meanwhile available for ID applications. Thus, in the present work, the focus of research is on the ID spraying of bond coats (BC) and thermal barrier coatings (TBC) for high temperature applications. An HVOF-ID gun IDCoolFlow mono with a N 2 injection was used to spray dense BCs (MCrAlY). The TBCs (YSZ) were sprayed by applying an SM-F100 Connex APS torch. Initially, flat steel samples were used as substrates. The morphology and properties of the sprayed ID coating systems were investigated with respect to the combination of different HVOF and APS spray parameter sets.

The currently available powders are for conventional high power HVOF or APS. There is an increasing need for low-power HVOF and HV-APS torches for coating temperature-critical parts and inner diameters. Using them for the new applications causes massive restrictions such as poor coating efficiency, much overspray, rough surface or defects in the layer structure. However, just using ultrafine powders does not solve the problem. The disadvantages in terms of fluidity and oxidation resistance need to be solved, too. Beside the development of modern HVOF, ID-HVOF and Three-cathode-APS coating systems Thermico pursues a continuous enhancement of customized powders. Due to this steady development, Thermico has realized spraying inner diameters with just 10 – 20 mm stand off by using a new WC carbide powder which is able to absorb enough energy out of the low power flame to build up a very dense high quality coating. The modified WC Co 83 17 powder is available for ID HVOF. The high quality coating of outer diameters with a HV-APS is another solution which Thermico has successfully developed. Beside this Thermico had the goal to establish a high quality HV-APS coating to prices competitive to HVOF.

Automotive, aerospace, and energy applications demand for reliable coating systems to enhance the operating efficiency and the lifetime of processes, machines, and components. HVOF sprayed WC-CoCr coatings are commonly used, especially for wear resistant applications. Due to their high hardness and adapted corrosion resistance WC-CoCr coatings show perfect preconditions for highly stressed tribological systems. However, dynamic loads, caused by vibrations, alternating temperatures or cycling are challenging issues. Fatigue cracking and delamination can occur, resulting in fatal damage of the coated component. Therefore, crack and fatigue resistant high performance coatings are needed. In this research work, the influence of the substrate pre- and post-treatment (grit-blasting and micro-finishing) on the fatigue behavior of warm sprayed WC-CoCr is investigated. It was determined that the fatigue behavior of the applied coating can be improved by micro-finishing. The smooth surface structure results in a low interface roughness. This significantly reduces notching effects under load and enhances the fatigue strength of the specimen.

Applications such as landing gears and turbine blades place new demands on near-net coating technologies. Such demands include the replacement of traditional grinding, finishing and grit basting techniques with better, more efficient methods. A method is described for near-net-shape spraying of complex internal and external geometries which eliminates the need for grinding. This is achieved by combining automatic, mass-flow controlled HVOF grit blasting with Nano-HVOF methods. The resulting coating displays an as-sprayed surface roughness of less than 2 μm Ra and a tight control over coating thickness and distribution. By carefully controlling the coating thickness and surface properties, it is possible to hone the required dimension and surface roughness.

This paper describes an inner diameter HVOF spraying technique and associated tests methods. In the experiments, WC-CoCr was applied to an internal diameter of 150 mm and evaluated using a dedicated pin-on-disk wear test and standard axial fatigue tests. The results are presented and discussed along with the potential for further development of the inner diameter HVOF spraying technique.

Demands on functional coatings with high dimensional accuracy and high surface quality has led to increasing interest in processing of very fine powder grades in a particle size range < 25 µm. Fine powders are not only showing a distinct potential for application of thin and dimensionally accurate coatings, but are also very promising for the production of dense and homogeneous coatings with improved mechanical properties. The large specific surface of fine powders is allowing for relatively low thermal energy levels that are introduced into the process. Nevertheless this also requires a very sensitive temperature control, to prevent overheating of the particles. The reduction of the thermal energy level is resulting in significant advantages particularly for the usability of the HVOF process for coating of inner diameters. Within this work in-flight particle properties of ultrafine carbide powders were analyzed. The studied HVOF process allows the adjustment of a broad parameter range by utilization of a hydrogen stabilized liquid fuel combustion process. A conventional straight nozzle type as well as a curved nozzle for internal spraying was studied. For a further assessment of the potential of ultrafine carbide powders also spray trials with a plasma spraying system have been made.

HVOF spraying of carbides is a well established method to produce high performance wear and corrosion protection coatings for various industrial applications. Using ultrafine feedstock powders with particle sizes well below 25 µm can offer many advantages with respect to the resulting coating properties, such as improved coating density and homogeneity and high dimensional accuracy. In this study, fine powder grades of WC 10Co 4Cr, WC 17Co and Cr 3 C 2 25(Ni 20Cr) in a particle size range of –15 +5 µm, -10 +3 µm and –5 µm have been sprayed with suitable HVOF systems on outer surfaces as well as on internal diameters. Resulting coatings have been analysed with respect to coating structure and microhardness. The coatings featured a favourable microstructure and very good hardness values. Furthermore, the Cr 3 C 2 25(Ni 20Cr) material was applied as wear protection coating on piston rings. The coating performance was evaluated in engine tests and proved to be significantly better than a conventionally graded standard material system for this application.

One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. In this work Cr 3 C 2 75-NiCr25 coatings were obtained by high-velocity oxy-fuel system from three agglomerated feedstock powders with various powder size distributions (- 30, -10 and –5 µm). The powders were agglomerated, sintered, plasmafused and crushed, in order to increase their density. The coating microstructures were characterised by SEM microscopy. Differences in coating roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the coatings obtained with the lowest feedstock powder size presented the best sliding wear resistance under all the conditions. This fact could be explained in terms of differences of cohesion between the carbide particles and the binder phase. The lower feedstock powder presents a lower carbide particle size that involves a better distribution and cohesion of the chromium carbide with the NiCr binder phase. This fact leads a minor production of “third body” hard particles in the wear tests that influenced quite considerably in the final wear rate of the studied coatings.