Many Engineers will be familiar with the methods available to provide a barrier against corrosion and wear but how many are familiar with the HVOF process? The Metalspray Industry has been established since the early 1900’s with process development leading us through Flamespray, Arcspray, Plasma spray and now to HVOF.
There is always a demand for more wear and corrosion resistant surfaces and for these reasons High Velocity Oxygen Fuel (HVOF) spray systems are becoming increasingly popular. Unlike the other methods of Metalspraying, where the feed stock is melted and projected onto the substrate, the HVOF simply soften the powder before projecting it. The main difference –which provides the superior quality- is offered by the use of a combustion chamber and accelerating nozzles that produce very fast (approx 1500m/sec) particle velocity that results in high impact energy and hence reduced porosity levels as when compared with other metalspray processes.
HVOF Flame with shock diamonds producing the necessary speed for high impact deposits.
The use of low level porosity coatings combined with hard wearing, corrosive resistant, conductive and high bond strengths allow the process to be used in many applications for a variety of Industries including Paper, Pump, Aerospace, Mining, Oil etc.
The coating density for most metallic coatings will be more than 99.5% of the theoretical density, micro hardness in excess of 1300 HV300 are commonplace and the bond strengths are beyond the normal values measurable by the ASTM 633 test.
HVOF Coatings - Harder, Denser & More Ductile
High Velocity coatings produce not only harder, denser carbide coatings, but are also more ductile. Due to the lowered particle temperature there is less shrinkage and this combined with a simultaneous peening effect as the next particle arrives, produces lower residual stresses and enables a much greater thickness to be applied. These properties make the coatings significantly more wear resistant, especially where loads are high or erosion is prevalent. Normal Plasma type coatings normally fail due to the break up of the coating and not through wear of the particles. A resistant barrier layer provides the corrosion resistance of the alloy materials, hence the reason why coating density is so important. High Velocity Systems ensure there is no degradation
Benefits of HVOF with Typical Coatings
Tungsten Carbide - If this material is overheated, not only does it oxidise and lose carbide to carbon dioxide, but the carbide also becomes dissolved with in the Cobalt Matrix. The resultant mixed Tungsten/Cobalt Carbides are hard, but brittle. In some circumstances this is not very important.
For example, when impact loads are low and wear is by stress abrasion. On the other hand, in applications such as tool and die parts and compressor blades subject to hammer wear, ductility as well as harness is required.
- MCrALY’s - are a family of high temperature oxidation resistant alloys which are used in gas turbines. They offer protection by readily forming an adherent oxide film on their surface, which delays further oxidation. However, this readiness to form oxides means that normally coatings will contain oxidised particles. Research has shown that these oxide layers around the particles act as a path for further oxidation and penetration of the oxidising medium to the substrate. The lower the oxide contents of the coating, the better. Oxidation rates follow a parabolic law. In other words, if the temperature is doubled the oxidation rate goes up by four times. Thus, if the particle temperature can be kept lower the oxide content is significantly reduced.
- Corrosion Resistant Alloys, eg Inconel 625 - These alloys provide corrosion protection by providing a resistant barrier layer. Therefore, coating density is very important and once again, oxide layers form paths along which the corrosion medium can migrate. Therefore, once again, a low temperature, high velocity process is appropriate.
HVOF - Typical Applications
As would be expected from such a coating system, the applications are wide and varied for instance:-
As a replacement for Hard Chrome Plating- for many years, hard chromium plating has been widely used in fluid handling and hydraulic applications. It has been proven to give adequate wear resistance in many cases and to provide an appropriate surface against which fluids may run. However, many situations exist where performance can be radically improved, whether in terms of pure wear or in combination with corrosion resistance. Thermally sprayed coatings are available in such a wide selection of materials and to such quality as to be able to exceed the requirements of almost any hard chrome application. The health and safety aspects of plating with hexavalent chromium have also been an increasing cause for concern. So much so, that some institutions intend to ban its use entirely. Thermally sprayed coatings do not produce effluent that is difficult to dispose of and the plant involved does not contain thousands of litres of toxic chemicals. It is also an ‘in-line’ process rather than a batch process so that material is only used or stocked during production. A further point worth noting is that the size of component for thermal spraying is not limited by the size of the otherwise plating bath.
Other Plating operations can also be replaced, eg Copper can be sprayed for inking rolls in the Printing Industry as well as providing excellent electrical conductivity properties for a variety of applications.
Aluminium can be sprayed very densely. Coatings of 7mm have been achieved and subsequently finely machined.
Pump components either as OEM or repair can be given enhanced lifetime and improved performance using a Nickel/Chrome/Iron/Molybdenum.
Gas Turbine Compressor Seals and Automotive Turbochargers benefit from the application of an Aluminium/Polyester Powder Blend applied by the High Velocity Process.
Valves and pumps subject to erosive wear from particles such as sand or ash etc can be protected using Tungsten Carbide/Cobalt/Chromium materials resistant to both wear and corrosion and in special circumstances Stellite 6 can be sprayed as a replacement of a weld clad overlay.
Incinerator firewalls are protected from corrosion at medium temperature by coatings of Inconel 625 and superheated tubes by Chromium Carbide in a Nickel based matrix.
As more and more Engineers become aware of availability and capability of High Velocity Systems then the applications for the process will surely grow well beyond its existing Market. Our video presentation below demonsrates the Hipo Jet 2700 HVOF system coating a pipe using MSSA HV 40/60.
If you would like more information on our equipment, or consumables please call us on 07 3823 1004, or email us using our contact form.