Illustrations of the need for precision and repeatability in tribology testing

Transcription

1 Illustrations of the need for precision and repeatability in tribology testing Falex Tribology nv dr. ir. Dirk Drees Tribology is the science and technology related to friction, wear and lubrication

2 Introduction : testing the real world in the lab Challenges - Cases

3 Introduction Testing is costly : lab tests are needed correlation cost time after Czichos, ASM Metals Handbook Vol 18 3

4 Lab testing strategy T.A.N. approach 4

5 Challenges and limitations to conventional lab tests Contact pressure Simulation pin on disk 1 N load on hard coated disk, 10 mm diameter ball Contact pressure : GPa range! Components Typical : MPa range Wear rate Machine component lifetime : > 2000 hr Typical tolerance loss (= acceptable wear) : ~ 2 µm ( or 2000 nm) Wear rate = 2000nm/2000hr = 1 nm/hour Typical bench test test duration : 1-10 hr Typical wear damage (= measureable) : ~ 1 µm (1000 nm) Wear rate = 1000 nm/10 hr = 100 nm/hour 5

6 Challenges and limitations to conventional lab tests Confidence => Statistics normal (infinite) distribution : average, standard deviation σ (unknown) Lab tests = sample of infinite distribution S : error on average not a measure for distribution width Confidence level T-distribution 6

7 Wear Volume Challenges and limitations to conventional lab tests Fn.d V 285 0, , , , , , , , , , , , , , , % confidence; 4 points 99% confidence; 15 points Load x distance (N.m) 7

8 Wear volume (mm 3 ) Challenges and limitations to conventional lab tests Wear evolution and low wear Run-in wear vs. Long term wear? Long term wear determines lifetime Run-in wear determines subsequent evolution (On-line) wear evolution measurement Measure low wear? see wear rate W Run-in 1,0 0,8 0,6 Evolution of wear volume in POE-0 in CO 2 (10 bar) in air 0,4 0,2 (log)t 0, test time (min) 8

9 Challenges and limitations to conventional lab tests Wear measurement precision and false results by poor resolution misleading results Example : Four Ball Wear test results Optical : Optical : 446 µm 405 µm 3-D Confocal : 3-D Confocal : 444 µm 270 µm 9

10 1. Low and online wear High energy proton beam produces a thin layer of radiotracers 2 measurement methods: direct and indirect - concentration measurement = best precision On-line wear measurement of engine parts - piston rings, cylinder walls, bearings, camshafts, turbochargers, - by TLA ON-LINE ENGINE WEAR MEASUREMENT TLA METHOD 10

11 1. Low and online wear Two challenges in one : Measure enough data Efficiency challenge Economic necessity Measure precise data Equipment challenges Danger of false results Typical wear rates in engine : nm/hour 11

12 Speed(rpm) Load (lbs) 1. Low and online wear Friction force (lbs) Conforming Block on Ring simulation of sliding journal bearing (e.g. crankshaft) TAN code 1519 realistic contact pressures (1-10 MPa) Materials Block : Brass Ring : SAE steel ring Falex S-25 Both activated : Zn-65 and Co-57 Lubrication : flow through chamber TLA detector engine base oil engine base oil + GMO-type friction modifier Tests Constant speed load : low speed (200 rpm, 30 lbs) Variable speed : Stribeck behaviour ( rpm) Load variation Reference experiment test time (s) 12

13 TLA wear particle detection (µm depth) 1. Low and online wear TLA wear particle detection (µm depth) Reference test Online wear of ring and block Oil + GMO friction modifier Online wear of ring and block 5,0 4,5 4,0 3,5 Speed (rpm) ring wear block wear ,0 4,5 4,0 3,5 Speed (rpm) ring wear block wear ,0 2,5 2,0 1,5 1,0 0,5 0, test time (s) 200 3,0 2,5 2,0 1,5 1,0 0,5 0, test time (s)

14 1. Low and online wear online wear measurement (µm) load in contact (lbs) TLA wear particle detection (µm depth) Speed High to low speed experiment 2 load levels : lbs repeats Macroscopic wear sensor false increase at each load increase TLA online wear Real increase at low speed & 45 lbs load = boundary lubrication Realistic wear depth ,8 0,6 0,4 0,2 0, ring wear block wear test time (s) , test time (s) 28 14

15 2. Improve friction precision Cylinder-piston assembly Fuel injector Source: Wikipedia Al alloy - Good heat transfer Reciprocating sliding conditions, mostly line contacts Source: ASM handbook, vol 18

16 2. Improve friction precision Conventional method TE77 ASTM G Cylinder-on-flat test: low carbon steel vs. 16 x 6 mm cylinder 100 N load (220 MPa) Two speeds: 2 Hz and 20 Hz (0.04 and 0.4 m/s)

17 RMS CoF 2. Improve friction precision RMS coefficient of friction At 2 Hz test frequency GMO PGMO A At 20 Hz test frequency GMO A PGMO C 80 C 120 C C 80 C 120 C Test duration (s) Test duration (s) Individual differences between GMO and additive A can be measured but the signal is noisy and requires statistical evaluation. Unclear fluctuations

18 2. Improve friction precision Piston simulator Use production piston rings and production engine cylinder (segments) Typical load 30 N load = 100 MPa Speeds: 5 Hz-10 Hz-30 Hz

19 2. Improve friction precision Piston simulator

20 % reduction in comparion with Base oil 2. Improve friction precision Differences more visible at slow speeds!!! GMO DGMO A (distilled) Load * Frequency (V.s -1 )

21 2. Improve friction precision F t F N Cantilever Light load/pressure Controlled speed High friction sensitivity 1. Sample 2. Counterbody 3. Optical sensor 4. Cantilever 5. Reciprocating table High precision microtribometer

22 2. Improve friction precision Ball-on-flat, AISI E steel, 25 mm x 8 mm disk, mm Ø ball 50 mn load (240 MPa), 2 mm stroke length, 0.5 mm/s speed, and 50 reciprocating cycles Repeatable test samples!

23 Average coefficient of friction 2. Improve friction precision 0,25 0,20 +GMO +B +C REPEAT MEASUREMENT +D Base +GMO oil +B +F +C +D Base oil +F 0,15 0,10 0, Number of cycles

24 Average coefficient of friction Average coefficient of friction 2. Improve friction precision 0,30 0,25 0,20 10W40 10W40 +GMO +GMO +B +B +C +C +D +D Base Base oil oil +F +F 0,15 0,10 0,05

25 2. Improve friction precision in conventional pin on disk tests Optimize test equipment for friction meas. Improved sensor Improved drive

26 2. Improve friction precision in conventional pin on disk tests CoF CoF Cylinder on flat contact Rotating disk Boundary lubrication N test, no solids N test, w ith solids Objective: Effect of chemistry and presence of solid particles on friction Time (s) Time (s)

27 3. Low wear, statistics and efficiency : multistation testing

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29 3. Low wear, statistics and efficiency : multistation testing Pin Uncoated DLC_TR PVD-DLC+H Graphite DLC PVD:H IBAD-DLC Uncoated PVD DLC PVD DLC IBAD DLC individual tests, 4 coating categories, wear evolution and total wear after 2.5 M cycles Duration project : 24 days = more than one result per day despite 24 day wear test! 29

30 3. Low wear, statistics and efficiency : multistation testing Wear damage of UHMWPE against implant surfaces bovine serum 1 Hz test frequency, 3.6 M cycles, 3 month test duration Immediate comparison of materials, roughness and batch reliability 40 test samples simultaneously tested 4 time intervals = 160 test results 30

31 CONCLUSIONS Tribology tests in the lab require a good description of reality Translate to lab machine Parameters to match wear and friction mechanisms Repeatability is an issue Main challenges in lab testing for realistic industrial applications Precision friction and wear Low Wear Rates Reliability and statistics Useful tools Thin layer activation technology for low and on-line wear measurements Microtribometer and high precision friction test equipment Multistation equipment for statistical results low wear rates Confocal microscopy for better wear evaluation Falex Tribology Wingepark 23b B-3110 Rotselaar +32 (0)

32 Thank you