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BASIC TECHNOLOGICAL DEVELOPMENT OF ADVANCED MATERIALS PROCESSING TECHNIQUES USING VACUUM PLASMA SPRAYING (VPS)

Objective


Vacuum plasma spraying (VPS) offers several advantages over conventional air plasma spraying (APS), eg, coatings with higher density and improved adhesion.

While testing the degree of adhesion of coatings to substrates, two new promising methods for ranking adhesion/cohesion were developed: shear testing and a modified scratch test. Direct current polarization techniques were used to evaluate the corrosion properties of metallic coatings, while alternating current (AC) spectroscopy was used to characterize ceramic coatings. Both methods provided information about the effects of porosity and on the interface of corrosion behaviour. The electrical properties of alumina coatings were fully characterized.

In general it was found that the present generation of plasma powders are not particularly suited to VPS applications. However, fully optimized coatings can have properties approaching those of the bulk materials, eg, the corrosion resistance of titanium and IN625, and the erosion resistance of alumina.

Process parameters were optimized by factorial experimental design for selected metallic and ceramic materials including titanium alloys, IN625, aluminium alloys, alumina ceramics, titanium boride and tungsten carbide cobalt cermets. Results were assessed by analysing the extent of melting of isolated particles caught on a polished surface and from the deposit efficiency. The factorial experimental design was found to be a very effective method for optimizing plasma spraying process parameters.

The following conclusions were reached:
laser diagnostics have shown that particles in VPS flames acquire significantly higher velocities (but with reduced dwell times) than particles in APS flames;
it is possible to obtain better phase control in VPS deposits; metals are free of oxide and there is much less degradation of tungsten carbide in tungsten carbide cobalt cermets than in other thermal spraying processes; ultra rapid quenching allows the retention of the m icrostructure of amorphous aluminium alloys;
the great kinetic energy of the particles yields coatings with much improved cohesion and adhesion; bond strengths in excess of 70 MPa have been measured for both metal and ceramic coatings; fully optimized coatings can have properties approaching those of bulk materials;
VPS coatings may contain very high levels of residual stress which can lead to failure; control of substrate temperature during the deposition process can minimize such stress.

Vacuum plasma spraying (VPS) is a relatively new technology used for the production of thick coatings (typically greater than 50 mm). The technology was developed from the well established air plasma spraying (APS) technique. The main difference is that the plasma is formed in a controlled atmosphere, usually argon at reduced pressure (typically 100 mbar) rather than the ambient atmosphere. The advantages of VPS over APS are perceived to be : less turbulent plasmas leading to higher particle velocities; purer plasmas, particularly low oxygen contents, and hence reduced particle oxidation; and the ability to sputter clean and heat the substrate using a transferred arc. The potential for controlling the technology using quantitive criteria has been demonstrated in preference to the preexisting empirical intuitive techniques.

A database on vacuum plasma spraying (VPS) was prepared including precursor powders, deposition conditions, analytical methods and the physical and chemical properties of the coating. This database was used to set initial starting conditions for optimization procedures. One of the most significant factors affecting coating development is the degree of particle melting since this influences the splat morphology, the level of porosity, interparticle bonding and adhesion grain size. Parameters affecting plasma melting were plasma gas flow, powder delivery rate, arc current, standoff distance, chamber pressure and powder injector gas flow rate. These parameter effects were adjusted so as to maximise the response (deposition efficiency). Materials studied included alumina, Inconel 625, tungsten carbide with cobalt, amorphous alloys, titanium, molybednum and tungsten.

Different test methods have been applied and compared with each other to study the adhesion of vacuum plasma sprayed (VPS) coatings to their substrates and to characterize their mechanical properties. These test methods include pull off test, shear test, hardness indentation test, fracture mechanics (4-point bend test, double torsion test, double contile beam test, scratch adhesion test) and residual stress measurement (deflection beam, X-ray sin{2} psi). The pull off test, shear test and hardness indentation test all showed that VPS coatings were better than air plasma sprayed (APS) coatings. With increases coating thickness these tests showed that the adhesion strength increased due to decreasing residual stresses caused by heat treatment of the sample. The remaining tests showed that the VPS coatings were of high quality which for the low loading test were as good as for bulk material.

Electrical measurements were made on vacuum plasma sprayed (VPS) coatings and compared with air plasma sprayed (APS) coatings and sintered ceramics for potential use as electronic packaging. Properties investigated included dielectric constant, dielectric strength, leakage current and the ability to screen print active components and conducting tracks onto the surface using standard techniques. The need for a high thermal conductivity component lead to the choice of a copper substrate for evaluation.
An attempt was made to use impedance spectroscopy to investigate the structure of the coating and the nature of the interface between the coating and the substrate. Measurements were performed with samples immersed in electrolytes of various compositions. Thickness and porosity were used as the primary correlation factors with the electrical characterization.
The VPS alumina coatings were found to be superior to APS coatings in their electrical, porosity and screen printing properties.

Direct current (DC) electrochemical techniques have been used to characterize sprayed metallic coatings with regard to their corrosion properties and to compare the corrosion properties with those of bulk materials of similar composition. The DC and alternating current (AC) electrochemical techniques have been used to characterize ceramic layers on metallic substrates mainly with regard to the content of interconnected pores. The electrochemical techniques have been used in the optimization of the spray process where the coatings are applied primarily as a corrosion protection. The results obtained were compared to the results of more traditional methods of evaluation of coatings, such as metallographic investigations and hardness measurements. It was found that the vacuum plasma spraying (VPS) technique allowed for the production of metallic coatings with good corrosion properties due mainly to absence of oxide containing phases and to low porosity.

The main objectives were to compare the wear properties of air plamsa sprayed (APS) coatings and vacuum plamsa sprayed (VPS) coatings under different temperature conditions and to correlate wear behaviour with important deposition parameters and hence particle characteristics.
To obtain maximum coating quality it is necessary to understand the interrelationships between processing conditions, particle behaviour and coating performance.
A laser diagnostic system (LDS) has been used to interrogate particle behaviour in the VPS system, in particular temporal and spatial velocity and density distributions. Results have been compared with friction and wear measurements made using a test rig development to allow room and high temperature tests to be performed under vacuum or controlled atmosphere.
THE PRINCIPAL OBJECTIVES ARE TO QUANTIFY THE VPS PROCESS, TO DEVELOP IT FOR A RANGE OF ADVANCED MATERIALS AND APPLICATIONS AND TO ENCOURAGE ITS EXPLOITATION BY EUROPEAN INDUSTRY.
TO MEET THESE OBJECTIVES IT IS AIMED TO PERFORM IN SITU INVESTIGATION OF THE PLASMA PROCESS IN PARALLEL WITH DETAILED STUDIES OF THE RESULTANT COATINGS INCLUDING THEIR MICROSTRUCTURE, PHYSICAL PROPERTIES AND PERFORMANCE UNDER MECHANICAL AND ENVIRONMENTAL TESTING USING A WIDE RANGE OF PROCEDURES AVAILABLE TO THE PARTNERS.

THE USE OF VPS AS A POTENTIAL METHOD FOR NEAR-NET SHAPE FABRICATION WILL ALSO BE INVESTIGATED.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

United Kingdom Atomic Energy Authority
Address
353,Harwell
OX11 0RA Didcot - Oxfordshire
United Kingdom

Participants (3)

ATV Korrosionscentralen
Denmark
Address

2600 Glostrup
Fraunhofer-Gesellschaft zur Förderungder Angewandten Forschung e.V.
Germany
Address
Lesumer Heerstraùe
28717 Bremen
Siemens AG
Germany
Address

51425 Bergisch-gladbach