What Does a Galling Test Programme Actually Need to Deliver?
Understanding metal galling is one challenge. Proving how materials and surfaces behave under controlled galling conditions is another entirely. As covered in our articles on metal galling and galling prevention methods, material selection, surface treatment and design geometry all influence galling resistance. However, none of those decisions can be validated by engineering judgement alone. They require test data — generated to a defined specification, under controlled and repeatable conditions.
A galling test programme is therefore not simply a matter of rubbing two surfaces together and observing the result. It is a precision engineering exercise. The quality of the data it produces depends entirely on the rigour of the test specification that governs it.
Why Test Specification Matters
Galling is an inherently variable phenomenon. Research consistently shows that galling behaviour is stochastic — it does not follow a simple, predictable threshold. Two nominally identical specimens tested under nominally identical conditions can produce different results. Furthermore, small variations in surface finish, specimen preparation, load application and test environment all influence outcomes significantly.
This variability makes test specification critical. Without a well-defined, rigorously controlled test protocol, galling test data cannot be compared between programmes, between laboratories or between material combinations. Moreover, data generated to a poor or inconsistent specification has no engineering value — regardless of how many tests were run. Therefore, the test specification is not a starting formality. It is the foundation on which every other aspect of the programme depends.
The ASTM G98 Button-on-Block Method
The most widely used standardised galling test method is ASTM G98 — the button-on-block test. In this method, engineers machine a cylindrical button specimen and a flat block specimen from the materials under evaluation. They polish both contact surfaces to a defined finish. They then load the button against the block at a specified contact stress and rotate the button through exactly one revolution against the stationary block.
After each test, both specimens undergo visual inspection at 10x magnification for evidence of galling. The test repeats at progressively higher stress levels until galling occurs. The stress at which galling first appears — the threshold galling stress — defines the material’s galling resistance under those specific conditions.
ASTM G98 is well established and widely referenced. However, it has recognised limitations. The threshold galling stress is not a fixed material property. Research has shown that for type 303 stainless steel, the lowest recorded galling stress can be over five times smaller than the highest non-galling stress for the same material combination. Consequently, a single threshold value understates the statistical variability that real engineering applications must account for. Furthermore, the visual determination of whether galling has occurred introduces subjectivity — experienced engineers examining the same specimen may reach different conclusions.
Pin-on-Disc and Alternative Configurations
Where ASTM G98 evaluates a single contact event under progressively increasing loads, pin-on-disc testing — governed by ASTM G99 — evaluates wear and friction behaviour under sustained sliding contact. A stationary pin presses against a rotating disc under a defined load. Frictional force and wear volume are recorded continuously throughout the test.
This configuration provides different and complementary information to the button-on-block method. It captures friction coefficient evolution over time, wear rate as a function of sliding distance and surface degradation progression across a full test cycle. Moreover, it allows engineers to evaluate lubricant performance, coating durability and the effect of sliding speed — variables that the single-rotation G98 method cannot address. Therefore, a comprehensive galling test programme often combines both methods — using G98 to establish threshold galling stress and G99 to characterise wear behaviour under sustained sliding conditions.
Additional test configurations include line contact methods using crossed cylinders, ball-on-flat arrangements under ASTM G133, and load-scanning test methods. Each offers different contact geometry and stress distribution characteristics. The right choice depends on what the application demands — the contact geometry the test must replicate, the loading conditions in service and the specific failure mode being evaluated.
Specimen Preparation and Surface Specification
Specimen preparation is one of the most critical and most frequently underestimated aspects of galling test specification. Surface finish profoundly influences galling behaviour. Therefore, every specimen must be prepared to a precisely defined surface roughness, measured and verified before testing begins.
Machining method matters equally. Cut surfaces and ground surfaces behave differently under galling conditions — their different grain flow characteristics and residual stress states influence adhesive wear initiation. Furthermore, cleaning and handling protocols must prevent contamination of contact surfaces before testing. Oils, fingerprints and airborne particles all alter contact conditions and introduce variability that the test specification exists to eliminate.
For hard alloy testing — including cobalt-based alloys such as Stellite and nickel-based superalloys such as Hastelloy — specimen preparation demands additional care. Research has demonstrated that Stellite 6 shows no galling under G98 testing at contact stresses up to 950 MPa at both room temperature and 300°C — a result that reflects both the material’s exceptional galling resistance and the importance of consistent specimen preparation in producing reliable data. Achieving comparable repeatability in a materials research programme requires meticulous control of every preparation variable.
Load, Environment and Data Requirements
A complete galling test specification defines far more than the test method and specimen geometry. It must also specify the load range and increments, the test environment — temperature, humidity, atmosphere and lubrication conditions — the number of repeat tests required at each load level and the criteria for determining test outcome.
Environmental conditions deserve particular attention in hard alloy research. Galling behaviour in Stellite and Hastelloy alloys is strongly influenced by temperature. A test programme that characterises ambient temperature behaviour only may not represent service conditions in aerospace, energy or chemical processing applications — where elevated temperatures significantly alter surface chemistry and oxide film stability. Furthermore, the data output specification must define what measurements the programme must generate — friction coefficient, wear volume, surface roughness change, microscopy requirements and statistical treatment of results.
Consequently, the test specification is not simply a procedure document. It is an engineering specification in its own right — one that defines the conditions under which data can be considered valid, reproducible and fit for engineering decision-making.
Bespoke Test Rigs for Specialist Galling Programmes
Standard tribometer equipment handles standard galling test configurations well. However, specialist materials research programmes — particularly those evaluating hard alloys under elevated temperatures, extreme loads or specific contact geometries — frequently require purpose-built test equipment.
Bespoke galling test rigs allow engineers to define the contact configuration, load range, temperature envelope, speed control and data acquisition precisely around the requirements of the programme — rather than adapting the programme around the limitations of available equipment. Furthermore, a purpose-built rig integrates the instrumentation needed to capture friction, load, displacement, temperature and surface condition data simultaneously and in real time.
CNR designs and builds bespoke galling test rigs for exactly these applications. With proven experience in hard alloy testing — including Stellite and Hastelloy — CNR’s test rig engineering starts from the programme specification and builds outward. If your galling test programme demands more than standard equipment can deliver, that capability is where the conversation starts.
Note: This article is for general information only
