To test the bond strength of SHS717 coatings with Vickers hardness (HV300) of 1,100 kg/mm2 test specimens were bent 180. The coating did not crack or delaminate which underscores not only the high bond strength but also the high resiliency of the coating.

Transmission electron microscope micrographs of SHS717 alloy which has been heat treated at 700C for 10 min. (Left) "Ideal" microstructure (average grain size 25 nm), (center) HVOF coating with average grain size of 50 nm, (right) wire-arc coating with average grain size of 80 nm.


Although the microstructures of Super Hard Steel (SHS) coatings are coarser than "ideal" (i.e., average grain size 25 nm) and additionally contain isolated larger scale regions formed during solidification, they can still be classified as nanoscale, says Michael Breitsameter, vice president NanoSteel Co., Idaho Falls, Idaho. "What's remarkable about the SHS coatings, however, is that they were produced in air using offtheshelf thermal-spray technology." Another advantage is that their feedstock material is specifically formulated for HVOF (high-velocity oxygen fuel), plasma, and wire arc, and is physically identical to conventional feedstock, he adds. "This eliminates the spraying and handling problems normally associated with nanoscale particulate materials."

Spray studies have shown the SHS feedstocks exhibit a wide operational window with great latitude and forgiveness to spray angle and distance, says Breitsameter. "This is important for parts with complicated shapes and for manual field applications." And while other ceramic-based coatings may be harder than those of SHS, they can only protect surfaces as long as they remain adhered to the part. The resiliency or damage tolerance of SHS717 coatings combined with high hardness gives SHS coatings extremely high bond strengths to a wide variety of metallic substrates.

For example, the bond strength of a SHS717 wire-arc coating was measured using ASTMC611 bond pull tests. The bond strength onto 1018 bond plugs was measured for coating thicknesses of 0.02, 0.04, 0.07, and 0.1 in. At the thickness extremes (0.02 and 0.1 in.) the bond strengths were 12 and 6.5 kpsi, respectively. "The bond strength of the 0.1 coating is higher than most conventional materials sprayed at 0.015 in. thickness," says Breitsameter. Remarkable, he continues, considering the fact that the coating was applied with no intermediate bond coat applied.

To further test bond strength, 180 bend tests were performed. "During the bend test," explains Breitsameter, "the outside of the coating is put in tension which can easily exceed the bond strength causing cracking or delamination." The SHS717 coatings did not crack or delaminate which underscores not only the high bond strength but also the high resiliency of the coating which is noteworthy considering it has a Vickers hardness (HV300) of 1,100 kg/mm2. Additionally, after heat treatment, hardness rose to 1,200 kg/mm2.

 

Microhardness (kg/mm2)
of SHS717 coatings
Application
Material hardness
Coating hardness
 
method
Condition
(HV100)
(HV300)
 
HVOF
As-sprayed
1,000 to 1,200
950 to 1,100
 
HVOF
Heat treated
1,300 to 1,450
1,150 to 1,300
 
Wire-arc
As-sprayed
1,150 to 1,250
950 to 1,100
 
Wire-arc
Heat treated
1,200 to 1,400
1,150 to 1,250
 
Bond strength of SHS717 Coatings
 
Carbon
Stainless
Substrate
steel
steel
Copper
Aluminum
HVOF, 0.04-in. thick, kpsi
>13
>13
11.2
>13
Wire-arc, 0.02-in.-thick, kpsi
12
10.2
4.8
1.08