Adhesive Wear Test ASTM G77
Adhesive wear, (scoring, galling, or seizing) occurs when two solid surfaces slide over one another under pressure. Asperities, or surface projections, exist on all flat surfaces and even those polished to a mirror finish are not truly flat on an atomic scale. When two flat surfaces come into contact, they initially touch only at a few of these asperity points. Friction and wear originate at these points. When the surfaces are subjected to a compressive load, the asperities are plastically deformed and eventually welded together by high local pressure. As sliding continues, these bonds are broken, producing cavities on one surface, projections on the second surface, and frequently tiny, abrasive particles are shed which contribute to future wear.
The ASTM G77 test determines the resistance of materials to sliding wear. The test utilizes a block-on-ring testing machine to rank pairs of materials according to their sliding wear characteristics. This test is designed to replicate adhesive metal-to-metal wear such as a feed screw flight land contacting the barrel lining. Results are reported as volume loss in cubic millimeters for both the block and ring. Materials of higher wear resistance will have lower volume loss.
In extrusion or injection molding equipment, adhesive wear is the principal mode of wear when the resin being processed contains little or no abrasive filler. The rotating feed screw contacting the barrel liner will, over time, result in micro-welding of metal between surfaces and a loss of material. Factors that contribute to this type of wear include:
Straightness of screw and barrel
Alignment of drive, barrel, feed housing and screw
Feed screw design
Non-uniform barrel heating
Improper barrel support
Unsupported downstream components (dies)
High head pressure
There is a general misconception that feed screw hardfacing or hard coating in contact with a softer barrel liner will cause premature wear to the liner. The reality is that adhesive wear is directly dependent on the ability or tendency of mating materials to form micro-welds between each other. A contributing factor to wear is how well these materials resist abrasion that results from the micro-weld fracture occurrence.
In early 2007, Extreme Coatings commissioned the ASTM G-77 testing of many commonly matched screw/barrel materials. The test applied a stepped load to the point of seizure which ranged from 20,000 to 55,000 PSI (140 to 380 MPa). Please note that the loads necessary for this test are significantly higher than a screw flight contacting a barrel lining would ever experience. For the purpose of determining optimum compatibility between materials, their performance was evaluated as an “end result” after being subjected to severe conditions. The results are presented in the following graphs.
The bars correspond to the barrel liner material, blue for bimetallic (FeCr) and red for tungsten carbide (WC). Labels indicate which material was tested against the liner material. These materials are Molybdenum, Colmonoy 56 (Ni/Cr/B), Stellite (Co/Cr), XC4000 (Chromium carbide) and XC1000 (Tungsten carbide).
Volume loss of bimetallic test blocks was not significantly different between materials with the exception of Stellite. This indicates that increased barrel wear does not result from tungsten or chromium carbide in contact with a “softer” barrel liner.
In the graph of rotating ring volume loss, blue is a bimetallic liner and red tungsten carbide. The Colmonoy and Stellite materials show significant volume loss in this test.
Molybdenum is an element of medium hardness and is characterized as a refractory metal. With a melting point over 4700°F (2600°C), Molybdenum has high resistance to micro-welding which gives it superior adhesive wear resistance compared to standard nickel and cobalt hardfacing alloys. Molybdenum’s abrasion resistance however, is characterized as low due to a lack of carbide formation with in the deposit.
Tungsten carbide, containing the refractory element tungsten, like molybdenum resists micro-weld related fracture and as a result, also has exceptional adhesive wear resistance. The added benefit of tungsten carbide is its high hardness which protects against abrasion caused by fractured asperities during the normal adhesive wear process. From the data generated, it appears that better adhesive wear resistance of the feed screw flight land promotes better barrel liner life of the commonly used materials.
Block on Ring Test Apparatus from
“Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test”. Designation G 77 – 05 ASTM International.
Increasing load to failure
Mineral oil lubrication
Total cycles > 20,000
Seize load 20 to 55 ksi (140 to 380 MPa)
Surface Engineering 
