Cold welding due to Fretting ("Fretting test facility")

The facility enables the measurement of the adhesion force between two materials in cyclic closed contacts which are subjected to fretting. This means that pin and disc are oscillated in their contact plane with small amplitudes (50 <> 100 µm). "Cyclic contact" means that the contact between the pin and the disc is closed and opened for several (thousand) times.

After closing the contact, fretting is applied for a selectable duration. After stopping fretting, the contact is opened and at each opening the adhesion force, i.e. the force necessary to separate both in vertical direction, is measured. High resolutions downto 50 mN can be achieved. The test provides the tendency to cold welding by a plot of the adhesion force as function of cycles.

The device is PC-controlled. To the control parameters, e.g. static load, holding and opening durations, fretting amplitude and frequency can be selected. On-line-data acquisition - for each opening the adhesion force is available - offers to post-process data, e.g. for analysing maximum adhesion forces, running-in- or steady state behaviour, as well as endurance of coatings.

Image of Fretting Test Facility:

Front window opened, loading by electromagnet thrugh bellows, force measurement directly below disc inside the vacuum chamber, vibration free by air suspension and ion getter pumps, fretting applied by piezo actuator.

Detail of Tribosystem:

• Pin (mounted onto actuator housed inside the copper block)
• Disc mounted onto the force transducer

Measurement of

• Adhesion force at each opening
• Fretting frequency and amplitude
• Environmental Data (Vacuum)
• Optional (e.g. residual gas, user defined)

Postexamination by

• Scanning Electron Microscope (SEM)
• Energy Dispersive X-ray analysis (EDX), profilometry

Fretting test facility Characteristics

Competence in Cold Welding Effects

Within the long term co-operation "Space Materials Testhouse" set-up of special devices and know-how has proven to be able to:

• Simulate real contact situations
• Enable all typical contact modes including "static", " impact" and "fretting"
• Measure adhesion forces automatically at each unloading with sensitivities down to 1 mN
• Compare adhesion data to adhesive wear by use of a ultra-high-vacuum tribometer

Some recent topics:

• The conditions that lead to blocking of the anchor of a Black-Body-Calibration-Device were successfully reproduced by 'Impact test'.
• Model-testing of Nickel- and Silver-plated wires proved the latter to be susceptible to cold welding and therefore stiffening.
• The new "fretting facility" has proven the assumption that fretting is the most dangerous contact type. Adhesion forces were found to exceed the load, if fretting is applied. (See Fig.1 below.)
• Comparison testing showed this test devices to be the most sensitive in Europe.

The Space Materials Testhouse is therefore uniquely able to offer:

• Advice and data on cold welding (Data-Base) for material/coatings selection
• Assessment, if the ECSS-Requirements on cold welding are fulfilled

Comparison of adhesion forces as function of cycles for "static-", "impact-" and "fretting tests" between stainless steel SS440C and titanium alloy IMI834. The adhesion force (9.5N) under fretting exceeds the load (4N). (Lines refer to envelope, i.e. peak value of intervals of 100 cycles. In brackets the static load L is indicated.)

Cold Weld Data base

Based on the result of "General Validation Studies" a data base was initiated:

• Comparable due to test parameters related to material properties
• It covers the contact modes " impact" and "fretting"
• A Summary table shows available material combinations
• For each tests a detailed data sheet is available
• The summary table shows a "calissification" related to the severity of adhesion

Related documents:

• The test method is described in ARCS-in-house-specification.
• This includes also sample dimensions and derivation of test parameters
• The summary of all tests is divided in Fretting and Impact
• For each test a detailed data sheet can be opened (click on symbol)

Classification of material combinations:

• The summary tables are based on following classification:

Symbol	Adhesion force [mN]	Comment to adhesion	Comment to Use
	lower imit upper limit
	0-------------------200	No or negligible adhesion, noise of test	Use recommended
	201----------------500	Small measureable adhesion	Security measures to be undertaken
	501--------------5000	Strong adhesion	Direct use not recommended
	5001------------higher	Severe adhesion	Direct use not recommended

ColdWeldData