Simply stated, the conventional process is slow speed at high temperature while the HVOF process is high speed at low temperature. Because the HVOF spray is at low temperature, the carbides don't decompose, thus high wear resistance is achieved from the same elements which would otherwise not provide such performance. The high speed spray also quite naturally yields a coating low in porosity which again enhances wear resistance.

Process	Gas Speed (m/s)	Particle Speed (m/s)	Flame Temp
Conventional	750	above 350	20,000 F
HVOF	1250	above 500	5,000 F

MAHLE’s HVOF coating, code named MC 238, is typically specified for top rings of diesel and high performance gasoline engines. Relative to subsonically sprayed rings, the HVOF process offers the advantage of improved wear characteristics for both the ring and cylinder bore. It does this while still maintaining excellent scuff resistance, something that is not easily achieved with higher wear resistant coatings.

Additional advantages of significance:

• Good adhesion to base material negating debonding concerns.
• Excellent cohesive strength, such that delamination is not a problem.
• Affordable cost

Perhaps one of the most significant advantages of HVOF has yet to be fully appreciated. Namely that is corrosion resistance. The next generation of emissions legislation will become reality for the diesel industry. For compliance, it will be necessary to begin recycling exhaust gas back into engine air intake systems. This technology of exhaust gas recirculation "EGR" causes many problems, not the least of which is a significant increase of an old problem, abrasive wear. Additionally; however, new and serious concerns are joining in. Corrosion is on this list. Historically the challenge of piston ring coating development was much simpler: "Produce a material which would stay on the ring and not wear out, while not also wearing the bore." Now with corrosion concern added to the mix, things become much more difficult because some properties which retard one problem actually promote another. HVOF appears to contain a satisfactory level of all requirements and do so in the very nasty cylinder bore environment exacerbated by exhaust gas recirculation.

A "three-in-one" chart is given on the right. The uppermost bars indicate scuff resistance. In this case taller is better. The middle bars indicate ring face wear comparisons while the lower most bars indicate cylinder bore wear. In the latter cases, lower is better. Bore wear is regarded as more important than ring wear because once the bore finish is gone, engine performance is degraded. It can be quickly seen that HVOF is eclipsed only by PVD, however, when cost is factored in HVOF retains its attractiveness.

With the addition of this new product line, MAHLE adds to its already wide range of ring coatings providing a full product compliment of base materials and coatings for all engine applications and requirements.