Thermal sprayed coatings are already used for the wear protection of cylinder bores since more than five years in the automotive industry, primarily in Europe. The adhesion of the coating on the different possible substrates is a key factor for the success of this operation. This paper gives an overview of the different potential processes used for the surface preparation prior to coating deposition. The influence of different substrate materials will be discussed also. Aluminum casting alloys, magnesium base materials, cast iron and also steel substrates will be considered in this paper. The safety and economical aspects of the different processes are considered also. Further, the paper will give an update of the state of the art of the thermal spraying of cylinder bores and the important aspect regarding the saving of energy resource.

New advanced thermal spray technology allows providing wear resistant coatings on the cylinder surface of aluminum or magnesium engines. The obtained special surface topography after the finishing allows to decrease significantly the coefficient of friction and to decrease the fuel consumption of an amount of 2 to 4%. Engine tests on diesel and gasoline engines have confirmed the value of this technology regarding the aspect of energy saving. This coating technology is introduced since 5 years in Europe by the manufacturing of high power diesel and gasoline engines respectively. The combination of different MMC coating materials allows the development of new specific solutions for each type of engine. Coatings with improved corrosion resistance and abrasion resistance were also developed and are available now. A brief overview on other applications of thermal spraying in the automotive industry will be also given. The MMC coating shows interesting perspectives for its use in diesel engines equipped with EGR systems to reduce the wear of the cylinder bores.

A reduction of the size and weight of connecting rods can be obtained using the deposition of bearing materials by thermal spraying directly on the substrate. For this application several thermal spray processes are coming in competition. However the aspect of the metallurgical reactions during the coating deposition should be taken in count. The alternative of different technologies is presented with respect on the coating characteristics, the potential of industrialization and of the costs. The characteristics of the coatings are also strongly depending of the part geometry.

Plasma spraying at low pressure conditions (LPPS) is a well established thermal spray process with a broad variety of important applications in different industrial segments. The operating conditions for LPPS processes can vary in a wide range from only a few mbar up to typically 200 mbar which imposes different characteristics of the corresponding spray conditions and resulting coating properties. Thermal spray processes have been approved being suitable for integrated fabrication of various layers for SOFC components. Depending on the process conditions, different layers used as functional coatings of SOFC components such as dense electrolyte layers as well as porous electrodes can be realized using the LPPS technology. Due to the flexibility of these processes, an optimized performance of the application on different target materials and geometries is possible with an increased technological and economical benefit compared to conventional thermal spray techniques. This paper presents an overview on the general potential and spray conditions of the LPPS process and its application for the deposition of various functional layers such as electrolyte and electrodes for SOFC components.

Since about the year 2000, cast aluminum automobile engine blocks have been coated in production lines using internal plasma spraying technology. Using this approach, the coefficient of friction between the cylinder wall and the piston assembly can be reduced to 30% and oil consumption is reduced of a factor two in comparison with cast iron. The extremely low wear rate of the friction elements increases engine life and reduces maintenance costs. The fuel consumption of the cars can be reduced from 2 to 4% in the case of gasoline and diesel engines. The coating costs are strongly dependent on production volume. For high volume production, the costs can be similar to those for cast iron liners. This paper reviews results from laboratory and field tests evaluating the performance and cost efficiency of plasma sprayed engine block liners.

This paper describes the investigation of low pressure supersonic plasma jets as found in LPPS processes. The main objective is to develop and validate a two-dimensional axisymmetric mathematical model representing such flows. Due to the supersonic nature of the jet, insertion of a measurement probe leads to the appearance of a detached shock in front of the probe. Consequently plasma values are measured behind the probe-induced shock, namely the stagnation enthalpy ( h o ), the stagnation pressure ( p o ) and the static pressure ( p ). The first two values are taken from enthalpy probe measurements while the third value comes from a new technique. Combining these measurements, a new interpretation method enables the calculation of the free-stream supersonic plasma jet properties. The mathematical model is validated using the enthalpy probe measurements and the free-stream properties from the new interpretation method. Results show that the model does not predict a static pressure as large as the new interpretation method. The principal cause for this discrepancy is attributed to the LTE assumption which is questionable for a 40 mbar plasma jet. The modelling effort reported here confirms the need to develop more detailed mathematical models for low pressure supersonic plasma jets in the future.

The Protal process combines surface preparation using a laser and thermal spraying in one production step. The laser preparation is based on a photomechanical reaction induced by the interaction between a laser of high instantaneous power and a polluted surface. The mechanism of bonding and the coating-substrate interface are then changed in comparison with grit blasting resulting in a significantly reduced substrate roughness. This study is aimed at finding the optimal Protal process parameters for the coating adhesion of a Ni5%Al sprayed on Ti6Al4V and IN718 alloys. The parameters investigated are laser beam intensity, the time delay between the laser impact and the spray impact, powder feed rate, substrate roughness and temperature. A test plan including these parameters is analysed by means of a fractional factorial design of experiment method. The adhesions of the coatings are measured using the ASTM C633 standard test. Data are analysed by a multiple linear regression model using a least squares fit. In addition, the coating/substrate interface is examined by optical and electron scanning microscopy (SEM) techniques as well as by Auger electron spectroscopy. Substrate roughness, substrate temperature and laser intensity are all shown to have a negative correlation with adhesion strength within the investigated range. Areas of diffusion are noticed at the coating/substrate interface.

For the deposition of functional coatings in cylinder bores of automotive engines different thermal spray processes are reviewed. Extensive studies of the process reliability have shown that the internal plasma spraying can be used in the industrial area of the automotive industry. Already today this technology is introduced in Europe and major Japanese companies are in an advanced status of evaluation. This paper describes the main processes used for these engine cylinder coatings and gives some results on typical coating characteristics and friction engine tests. It was shown in engine tests that the friction between piston ring and cylinder wall can be significantly reduced. Further the oil consumption can be significantly reduced and the wear resistance increased. The cost of the developed coating solution can be considered as low for this type of application.

Low Pressure Plasma Spraying (LPPS) is nowadays a well-established thermal spray process with a broad variety of important applications for functional surface coatings. The operating pressure for LPPS processes can vary in a wide range from typically 200 mbar down to only a few mbar. This leads to unconventional properties of the plasma jet, in terms of supersonic flow with strong shock structure at moderate pressure, towards rarefaction and frozen flow at very low pressure. In order to optimize and control the spray processes for specific applications, it is necessary to understand the underlying physical mechanisms. However, so far only limited knowledge has been established on the plasma jet properties and its interaction with the spray particles in LPPS conditions. We present several experimental investigations to characterize plasma spray processes under various pressure conditions. Measured plasma jet properties using a dedicated enthalpy probe system and imaging are combined with IR-pyrometry and velocimetry on the particles (DPV2000) to further improve the understanding of the plasma particle interactions. These results, along with spray deposit characterization, can be used to optimize the coating properties and explore further potential applications.

Coating of cylinder bores using plasma spraying is a growing application of thermal spraying in automotive industry. The Sulzer Metco RotaPlasma manipulator provides continuous rotation of the internal diameter plasma gun and enables a cost-efficient and reliable coating of the cylinder blocks. The development towards more advanced quality control systems and closed-loop spray control systems requires an on-line measurement of the most important particle and spray parameters such as particle temperature, velocity and flux. However, the small dimensions of the plasma plume of an internal diameter spray gun together with the rotation of the gun makes the measurement difficult, if not impossible, for many traditional diagnostics equipment. In this work, SprayWatch, an imaging CCD camera based diagnostics system from Oseir Ltd. was used for measuring particle and spray parameters during the rotation of an internal diameter plasma spray gun. The results show that the SprayWatch camera system is a promising tool for both development and production monitoring of cylinder bore coating process, as well as for realising a closed-loop spray control system.

This paper provides an overview of the tools and techniques used in industry to monitor and control plasma spraying processes. It describes how the tools work and what they can reveal about spraying processes and coating properties. It also presents examples showing how the tools are used to optimize spray parameters for special applications and discusses the possibility of implementing closed loop control for plasma spraying processes. Paper includes a German-language abstract.

The pore microstructure of thermal spray coatings can be described as a combination of interlamellar pores, intralamellar cracks, and volumetric globular pores. This complex pore morphology can strongly influence the properties of the deposited layer. In this paper, the authors use a multiple small-angle neutron scattering (MSANS) technique to characterize the pore system in NiCrAlY layers. It is shown that the inter-lamella system is of utmost importance. Paper includes a German-language abstract.

Different thermal spray processes are being used to provide coatings for cylinder bores in automotive engines. The internal plasma spraying technology described in this paper applies protective layers to cylinder surfaces in engine blocks made of AlSi cast alloys. This method enables a significant reduction in the coefficient of friction, a reduction in oil consumption, and an increase in wear resistance compared to cast iron, the standard material for cylinder liners. Paper includes a German-language abstract.

Chemical reactions between melted materials and their gaseous environment are generally inherent to the thermal spray process. Measures to promote and control these reactions are the distinguishing characteristic of reactive plasma spraying. In this paper, Ti-6Al-4V nitrided coatings are produced by high-pressure reactive plasma spraying. The coatings are deposited at different pressures up to 250 kPa in a reactive nitrogen atmosphere as well as air in order to study the influence of spraying pressure and atmosphere. The microstructure and phase composition of the Ti-6Al-4V layers are examined with the aid of X-ray analysis, microprobe measurements, and electron imaging. The investigations show that the pressure-supported nitrogen application during spraying led to the formation of fine and coarse TiN in the Ti matrix. Paper includes a German-language abstract.

This paper discusses the advantages of low-pressure plasma spraying along with recent improvements in LPPS technology. It also reviews current and emerging applications and examines the microstructure of different coating materials deposited using low-pressure plasma spraying techniques. Paper includes a German-language abstract.

This paper evaluates an electrochemical mapping method for determining the corrosion resistance and structural integrity of thermally sprayed coatings. In the test setup, a potentiostat is suspended over the test sample, forming an electrochemical cell. The circuit is completed through an electrolyte-containing porous tip. Capillary forces keep the electrolyte on the surface of the tip, preventing transfer to the substrate surface. In the investigation, electrochemical, spatially resolved measurements are carried out on flame and vacuum plasma sprayed nickel-base coatings and compared with the results of salt spray testing. It is observed that the new method offers many advantages being faster, nondestructive, and quantitative in nature. Paper includes a German-language abstract.

This paper investigates the effect of chamber pressure on plasma jet expansion characteristics. It presents images of the plasma jet corresponding to different chamber pressures and torch parameters and correlates them with enthalpy probe and pressure measurements recorded in different areas of the torch nozzle. A transition from an over-expanded to an under-expanded flow regime, as evidenced by a change in jet topology, is shown to be a function of chamber pressure. This transition pressure strongly depends on torch parameters and is characterized by an estimation of a rarefaction parameter based on nozzle exit and chamber pressure. At low chamber pressures, a progressive change from a continuum to a transition flow regime is shown by the thickening of the shock structures. Paper includes a German-language abstract.

This paper discusses the principles of laser shock adhesion testing, a nonintrusive method for measuring the bonding strength of coatings. The technique uses a laser to generate a thermal shock in the substrate, while Doppler laser interferometry is used to detect detachments. An application example is presented in which the method is employed to measure the adhesion strength of a plasma-sprayed copper coating on an aluminum substrate. The results are compared with numerical simulations of shock wave propagation. Paper includes a German-language abstract.

A numerical model of an argon jet exiting a LPPS torch has been developed and validated against enthalpy probe measurements for a slightly overexpanded jet at a chamber pressure of 100 mbar. Visualization of the jet using a CCD camera shows the presence of a small Mach reflection in the first compression-expansion cell with only oblique shock waves in the second cell. This jet topology is also observed in the model results. The images of the enthalpy probe on the axis of the plasma jet reveal that the shock layer, or shock-probe distance, varies according to the axial location of the probe. Shock-probe distance can be as large as 3 mm and should be considered when mapping plasma jets. Paper includes a German-language abstract.

This paper describes an experimental investigation of plasma jet properties of a DC torch operated at low pressure (below 10 mbar). A modified enthalpy probe system is described, which allows gas sampling from the plasma jet at pressures down to the mbar range. Measurements of the specific enthalpy, temperature and velocity throughout the jet for different pressures are presented and discussed. In the pressure range investigated, the jet flow is supersonic and compressible theory is used to infer the velocity from the dynamic pressure measured at the probe tip. In addition, optical emission spectroscopy of the plasma jet is used to evidence the differences of these low-pressure plasmas with respect to common, atmospheric pressure thermal jets. These preliminary measurements are the starting points towards a better understanding of plasma jets at low operating pressures in view of new process development and optimisation.