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
Introduction : testing the real world in the lab Challenges - Cases
Introduction Testing is costly : lab tests are needed correlation cost time after Czichos, ASM Metals Handbook Vol 18 3
Lab testing strategy T.A.N. approach 4
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 : 100-300 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
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
Wear Volume Challenges and limitations to conventional lab tests Fn.d V 285 0,251 570 0,822 855 0,909 1140 1,381 1710 2,438 570 0,922 1140 1,223 1710 2,383 2280 3,088 3420 4,405 855 0,776 1710 2,133 2565 3,533 3420 4,857 5130 8,414 10 8 6 4 2 0 99% confidence; 4 points 99% confidence; 15 points 0 1000 2000 3000 4000 5000 6000 Load x distance (N.m) 7
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 0,0 0 50 100 150 200 250 300 test time (min) 8
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
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
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
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 (1500-200 rpm) Load variation Reference experiment 250 200 9 8 7 150 100 50 6 5 4 3 2 0 1-50 0-100 0 100 200 300 400 500 600 700 800 test time (s) 12
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 7637 400 5,0 4,5 4,0 3,5 Speed (rpm) ring wear block wear 7638 400 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0 0 200 400 600 800 test time (s) 200 3,0 2,5 2,0 1,5 1,0 0,5 0,0 0 0 200 400 600 800 1000 test time (s) 200 13
1. Low and online wear online wear measurement (µm) load in contact (lbs) TLA wear particle detection (µm depth) Speed 50 7651 1600 High to low speed experiment 2 load levels : 30-45 lbs repeats 40 30 20 1400 1200 1000 800 600 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 10 0 0,8 0,6 0,4 0,2 0,0 0 5000 10000 15000 20000 25000 ring wear block wear test time (s) 7651 400 200 0 46 44 42 40 38 36 34 32 30-0,2 0 5000 10000 15000 20000 25000 test time (s) 28 14
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
2. Improve friction precision Conventional method TE77 ASTM G133... 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)
RMS CoF 2. Improve friction precision RMS coefficient of friction 0.16 0.14 At 2 Hz test frequency GMO PGMO A 0.16 0.14 At 20 Hz test frequency GMO A PGMO 0.12 0.12 0.10 0.10 0.08 20 C 80 C 120 C 0.08 20 C 80 C 120 C 0.06 0 500 1000 1500 2000 2500 3000 Test duration (s) 0.06 0 500 1000 1500 2000 2500 3000 Test duration (s) Individual differences between GMO and additive A can be measured but the signal is noisy and requires statistical evaluation. Unclear fluctuations
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
2. Improve friction precision Piston simulator
% reduction in comparion with Base oil 2. Improve friction precision 0-5 -10-15 -20 Differences more visible at slow speeds!!! GMO DGMO A (distilled) 0 5 10 15 20 25 30 Load * Frequency (V.s -1 )
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
2. Improve friction precision Ball-on-flat, AISI E 52100 steel, 25 mm x 8 mm disk, 3.175 mm Ø ball 50 mn load (240 MPa), 2 mm stroke length, 0.5 mm/s speed, and 50 reciprocating cycles Repeatable test samples!
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 0,05 0 10 20 30 40 50 Number of cycles
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
2. Improve friction precision in conventional pin on disk tests Optimize test equipment for friction meas. Improved sensor Improved drive
2. Improve friction precision in conventional pin on disk tests CoF CoF Cylinder on flat contact Rotating disk Boundary lubrication 0.14 5 N test, no solids 2 1 3 4 5 6 0.3 5 N test, w ith solids 8 7 9 10 11 12 0.12 0.28 0.26 Objective: Effect of chemistry and presence of solid particles on friction 0.1 0.08 0 50 0.24 0.22 0.2 0.18 100 0.16 150 200 250 300 Time (s) 350 400 450 500 550 600 0.14 0.12 0.1 0 50 100 150 200 250 300 Time (s) 350 400 450 500 550 600
3. Low wear, statistics and efficiency : multistation testing
3. Low wear, statistics and efficiency : multistation testing Pin Uncoated 2750 101000 835560 2494440 4 1 2 8 8 6 0 0 7 11 14 0 2 2 11 17 3 3 4 16 21 2 2 1 17 28 0 1 9 22 32 0 2 14 21 41 0 1 7 23 DLC_TR PVD-DLC+H 2750 101000 835560 2494440 2 1 1 1 5 10 0 0 2 8 26-1 0 2 3 35 1 1 2 1 44 0 1 3 9 46-1 0-2 2 Graphite DLC PVD:H 2750 101000 835560 2494440 16 0-1 1 9 19 5 4 0 1 23 1 1 7 17 30 1 2 3 8 37 0 0 1 6 49-1 1 3 19 IBAD-DLC 2750 101000 835560 2494440 8 1 2 19 31 12 0 2 8 15 24 2 0 8 13 34 0 1 10 28 39 0 4 5 12 43 0 2 15 21 35 30 25 20 15 10 5 0 35 30 25 20 15 10 5 0 Uncoated 0 500000 10000001500000200000025000003000000 PVD DLC 2 0 500000 1000000 1500000 2000000 2500000 3000000 35 30 25 20 15 10 5 0 35 30 25 20 15 10 5 0 PVD DLC 1 0 1000000 2000000 3000000 IBAD DLC 0 500000 10000001500000200000025000003000000 26 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
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
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)16 407 965 www.falex.eu
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