new unidirectional single pass wear testing procedure · new unidirectional single pass wear...

NEW UNIDIRECTIONAL SINGLE PASS WEARTESTING PROCEDURE

N. M. Renevier, S. Poulat and D. G. TeerTeer Coatings Ltd. (TCL)

290 Hartlebury Trading Estate,

Hartlebury, Worcestershire

DY104JB, U.K.

Abstract The ST-3001 is a multi-mode testing system which can be used for scratchadhesion tests, linear wear tests either reciprocating or uni-directional andhardness testing. The system was based on an earlier tester ST-2000 [1] andwas further developed in a join project supported by the European Commis-sion [2]. In conventional pin on disc or reciprocating wear tests, the sametrack is rubbed repetitively and such tests can be used to simulate the wearconditions for components but they do not simulate cutting or forming op-erations where new material is continuously bought into thecontact zone.Recently, the ST-3001 has been used to provide an initial assessment for suit-able coatings and substrate materials used in cold forming operations. Testshave performed at several loads under several environmental conditions (dryand lubricated) and a conditions for a new accelerated test have been es-tablished. The new testing procedure will be fully described in the paperand results will be correlated with industrial results. This is a powerful tech-nique for simulation sticking of gummy materials such as aluminium, copper,stainless steel, lead and zinc for forming operation.

This paper is an extension from the paper presented at ICMCTF2002 inSan Diego 2002.

INTRODUCTION

Coatings are commonly used in a wide range of industries, such as au-tomotive, aerospace, optics, construction, engineering and micro-electronicsectors. In most of these sectors, the use of coatings is expanding and there

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894 6TH INTERNATIONAL TOOLING CONFERENCE

is an increasing need to control coating specific parameterssuch as theirmechanical properties or adhesion between coating and substrate. Coatingtechnology is fundamentally dependent upon good adhesion between thecoating and the substrate, and in many cases adhesion is the limiting factorfor the wider application of the technology.

The scratch test is routinely used as a quick and simple tool to monitor theadhesion of a coating onto the substrate and is designed for the assessmentof the mechanical integrity of coated surfaces. This technique is well docu-mented for hard coating failure mechanism reconnaissance [3]. A Europeanpre-standard [4] has been accepted whose definite adoption is pending. Ballon disc tests and reciprocating tests are used to assess fatigue and wear re-sistance properties, whereas the ball cratering tests is used for testing theabrasion resistance. Those tests are based on the principlethat the twoparts in contact are rubbed repetitively at the same place [5], therefore thesetests are suitable for components but are less suitable for cutting or formingprocess where a new material is in contact.

Approximately 20% of the down time in the press shop are due togalling.Because most industries are relaying on ”just in time” and continuous pro-duction, any disruption has a dramatic economical effect. On the technicallevel there are, in addition, two major direct detrimental consequences.

a) Material loss owing to severe surface damage of the formedparts in thefunctional regions making them unacceptable for further assembling.

b) Critical build up of well-adhered, highly work hardened material fromthe workpiece material imperatively, requiring a reconditioning of thetool surface and leading to an accelerated wear of the tools.

Tools are expensive and almost always-unique tools generally intended tolast over the entire production period for a specific part. Therefore, gallingis an extremely serious incident. Thus galling prevention must be integratedfrom the very start of die and mould

conception. Therefore, there is a strong need for a ranking suitable coatingto prevent galling. Our approach starts with a laboratory simulation using anewly developed procedure on the ST3001 for surface damage assessment.

Owing to the formation of adhesive junctions, material transfer from thesofter friction partner (the sheet surface) to the die may occur. The severityof this junction formation is intimately related to the "chemical affinity"(mutual solubility) of the friction partners at the contactinterface. As a

New Unidirectional Single Pass Wear Testing Procedure 895

general rule, for metals, the interaction is strongest for friction partners ofidentical chemical composition. This adhesive interaction may be partiallyscreened by a "third body" [6] interlayer (surface oxide, contaminant film,lubricant, etc.) and will result in a more or less continuoustransfer layer.Clearly, in the case of discontinuous asperity build-up, especially if the lattercorresponds to severely work hardened material, local stress concentrationswill be produced, leading to the evacuation of screening interlayers and willresult in macroscopic junctions and, possibly, deep scars:This is galling.Therefore, engineers will aim for constant friction conditions with a lowadhesive contribution by the reduction of excessive local contact pressuresand "chemical affinity" optimisation of the friction partners.

THE DESIGN

THE OBJECTIVE

While indispensable as a final validation before press shop introduction,real scale simulation experiments are expensive and time consuming. Inmost cases, they are

inadequate for determining failure origin analysis. Therefore, a new pro-cedure has been developed on the ST3001 and tested (Fig. 1) asa laboratorysimulation experiment for the optimisation and the realisation of data baseof various tool/ workpiece combinations. In this test, a ball (coated or un-coated) will represent the working tool to be tested, whereas a flat samplewill represent the workpiece material to be formed by the end-use. Figure 2shows a bending application where this new procedure can be used.

The building-up of particle of particle on tools is particularly significantwith copper and aluminium alloys. These may results in premature tearingand/or scratching of workpiece in severely strained areas [7, 8]. To preventand delay the occurrence of the metal transfer and galling, three state of theart low friction coatings have reported (Table 1).

BASIC MOTIONS OF ST3001 MULTI-MODE TESTINGSYSTEM

Loading and unloading system The basic motions has been described in[9]. (Fig. 3). The load is applied by the spring to the ball shaft through thecantilever beam and the load cell. The spring is compressed by the bush


Table 1. Material parameters

ToolSurface finish Dimension

WC-4% Co Highly polished 5 mm diameter

CoatingsThickness Method Type

Graphit-iCTC 2.3-2.7 PVD CFUMSIP Me-CMoSTTM 1.0-1.2 PVD CFUMSIP MoS2 / Ti

Dymon-iCTC 2.5-3Combined PVD

CFUMSIP and PECVDDLC (-CH)

WorkpieceSurface finish Pre-cleaning

AISI 316L 1200 SiC solventAl 2014 1200 SiC solvent

which is moved by operating the servo motor. The movement of the motoris controlled via a load feedback loop assuring that the loadapplied on thesample is correct.

Translation system (Figure 3). The motorised translation table in thedirection perpendicular to the wear track direction is usedsolely to positionthe sample under the ball shaft. The motorised translation table in the weartrack direction is used to position the sample under the balland to providecontrolled motion during a test. The motorised translationtable is fittedwith a frictionless table, which acts as a sample holder. Thetable is lockedin place by four screws; these screws need to be in place to ensure thatthe sample holder rigidity is adequate for the test. The frictionless table islocked on the sample bed by two locking nuts and can be moved byusing theadjusting screw. In order to avoid wear of the rails of the translation table,this adjusting screw enables the operator to move the frictionless table alongthe rails so that the wear of the rail is more homogenous.


Data logging system The ST-3001 software package then analyses, dis-plays and saves the collected data.

New procedure principle The newly developed procedure could be de-scribed as a multi-single uni-directional scratch test with constant displace-ment and under constant load. A procedure detailed below hasbeen devel-oped to operate semi-automatically. The

procedure consists on a succession of loading-unloading and small dis-placements. The different sequences reported in Fig. 4 are the following:

Step 1 When the ball is at the chosen position, the ball is progressivelyloaded to 30 N.

Step 2 A 2 mm displacement has been chosen to simulate bending of smallparts, this distance can be adjusted for other applications.

Step 3 When the rubbing test has been completed, the ball is progressivelyunloaded.

Step 4 The ball is moving back at the beginning of the track where it ispossible capture a picture of the track and analyse the ball building upor wear.

Step 5 The ball is moved 1mm after the end of the previous track, a newtest can be performed.

Step 6 This step is identical to step 1.

SAMPLE PREPARATION AND POSITIONING

5 mm diameter highly polished uncoated and coated balls havebeen usedto simulated the tool (See Table 1 for more details), whereasthe metallicworkpiece materials (Table 1) have been polished to 1200 grain size with SiCpaper. Following extensive studies carried out as part of a European project(S, M & T Contract No MAT1-CT 940045), the ball (coated or uncoated) iscleaned before beginning of the test. The ball and the workpiece are wipedwith a soft tissue soaked with solvent to remove finger printsand a hair drieris used to evaporate the solvent. Before the test, uncoated and coated balltip must be kept free of fingertips. The workpiece material ismounted in themiddle of the frictionless table, whereas the ball is positioned on the edge of


the sample using the step positioning of the table and using the microscopefor final examination. It is possible to move the sample either using thestep positioning option as described above or directly to the microscope.Pictures of the workpiece material can be captured by the camera. It is nowpossible to add comments, scale and arrows on the picture. The damagedon the workpiece can be assessed and failure criteria can be imposed by theoperator.

RESULTS

The building-up of particle on tools is particularly significant with softalloys such as copper and aluminium alloys. These may results in prematuretearing and/or scratching of workpiece in severely strained areas [5, 6].

ALUMINIUM ALLOYS

Uncoated and coated (Graphit-iCTC [8, 9], MoSTTM [10, 11] and DLC)carbide balls have been rubbed against Al 5% Cu, where friction coefficient,and damaged on the ball have been reported. From Fig. 5, it canbe seenthat building up is occurring on the carbide and the Graphit-iCTC coating atthe beginning of the test. This results to a poor quality workpiece material(scratching of workpiece) and increasing friction coefficient from 0.15 to0.44 for the carbide and 0.49 to 0.60 for the Graphit-iCTC coating. On theother hand, the hydrogenated DLC coating has a very different behaviour, itwas not possible to detect any building up, small particles are incrusted inthe ball due to the surface finishing, this could be eliminated by using evenhigher quality polished balls. The surface finishing is an important factor inthe building up formation. The DLC coating retains a low friction coefficientthrough the test from 0.36 to 0.17. Regarding the MoSTTM coating, thesurface finishing of the ball is higher than the one of the workpiece material.The copper is able to create scratches with would led to creation of a buildingup with time. The surface finishing of the workpiece materialis higherquality at the beginning of the test, but deteriorate rapidly as the building upincreased. The friction coefficient is increasing through the test from 0.12to 0.49. From these test the use of DLC coating is recommended.

Stainless steel AISI 316L Uncoated carbide balls have been rubbedagainst AISI 316L, where damaged on the ball and on the workpiece ma-


terial have been reported. From Fig. 6, it can been seen that building up isoccurring on the carbide during the test. First there is accommodation of thetwo surfaces followed by debris accumulation. This resultsin scratching ofworkpiece and poor quality product.

CONCLUSIONS

While indispensable as a final validation before press shop introduction,real scale simulation experiments are expensive and time consuming. Inmost cases, they are

inadequate for precise failure origin analysis. A new procedure has beendeveloped and tested on the ST-3001 multi-mode testing

system as a laboratory simulation test for an initial assessment and optimi-sation of suitable system substrate / coating / workpiece materials / lubricantsused in cold forming operations. In our present work, the newprocedurewas tested for the simulation of sticking of gummy materialssuch as Al, Cu,Sn, Ti and stainless steel or other materials. The procedurecan be used dryor even lubricated for testing the suitability of new systems.

ACKNOWLEDGMENTS

The Authors would like to thanks Dr Juergen Von Stebut for theusefuldiscussions and the European Commission for the financial support througha Brite Euram project number SMT4-CT97-2150.

REFERENCES[1] V. BELLIDO-GONZALEZ, N. STEFANOPOULOS and F. DEGUILHEN, Surface

and Coatings Technology 74-75 (1995) 884-889.

[2] Multimode scratch testing project SMT4-CR1997-2150

[3] R. REZAKHANLOU and J. VON STEBUT, "Damage mechanisms of hard coatingson hard substrates : A critical analysis of failure in scratch and wear testing" .in"Mechanics of Coatings", Tribology Series 17, Eds. : D. Dowson, C.M. Taylor, M.Godet, Elsevier, Amsterdam 1990.

[4] Advanced technical ceramics - Methods of test for ceramic coatings - Part 3: Deter-mination of adhesion by scratch test, ENV 1071 - 3: 1994

[5] K. J. WALH, M. BELIN and I. L. SINGER, Wear 214 (1998) 212.

[6] M. GODET, Wear, 100 (1984), 437

[7] J. M. STORY, G. W. JARVIS, H. R. ZONKER and S. J. MURTHA, Issues and trendsin automotive aluminium sheet forming, SAE Publication no.SP-944 (1993) 1.


[8] W. R. D. WILSON, tribology in cold metal forming, J. Manufac. Sci. Eng. 119 (1997)695.

[9] N. M. RENEVIER, S. POULAT and D. G. TEER, Presented at ICMCTF 2002 in SanDiego, Ca, USA, 22-26 April 2002.

[10] D. G. TEER, D. CAMINO and A. H. S. JONES, UK Patent appl. GB9 725 413, 1997

[11] D. CAMINO, A. H. S. JONES, D. MERCS and D.G. TEER, Vacuum,52 (1999) 125.

[12] D. G. TEER, V. BELLIDO-GONZALES and J. HAMPHIRE, "MoS2/Titanium Coat-ings" UK Patent GB9514773.2 (19/07/1995), EU Patent 0842306

[13] N. M RENEVIER, J. HAMPSHIRE, V. C FOX, J. WITTS, T. ALLEN and D. G TEER,Surf. Coat. Technol., 142-144 (2001) 67.


(a) general view

(b) main software window

Figure 1. Teer Scratch and Wear tester ST 3001.


Figure 2. Schematic diagram of a bending process.


Figure 3. Loading system and translation system.


(a)

(b)

Figure 4. (a) Description of the procedure and (b) parameters for step2.


№ Uncoated Carbide Graphit-iCTC MoSTTM DLC

0

1

5

10

50

100

Figure 5. Aluminium-5% Copper Alloy- tool building up and Friction coefficient.


0 1 5 10

50 100 150 200

Figure 6. Stainless steel - WC tool building up.

new unidirectional single pass wear testing procedure · new unidirectional single pass wear...

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