Evaluation of surface coatings and layers by modern methods

Transcription

1 Evaluation of surface coatings and layers by modern methods Kříž A., Beneš P., Sosnová M., Hrbáček P. University of West Bohemia - Pilsen Department of Material Science and Technology 13th International Symposium on Metallography 2007, Stará Lesná, Slovak Republic

2 The components in many industrial applications are exposed to intensive effects of contact stress. Degradation occurs during the contact stress of two surfaces because of their interaction. Degradation the limited factor of lifetime It is necessary to find the ways of suppressing this degradation or eliminate it to an acceptable limit. Dynamic contact wear is not the basic kind of wear it consists of basic kinds of wear (adhesive, abrasive, fatigue and fretting wear). 2/25

3 The solution to this problem with the bulk material is very expensive and very often useless. The suitable surface modification is adequate to assure the necessary function. Surface properties is necessary to solve generally!!! It is not possible to prefer one property to the others and suppose that the problem is solved ( hardness resistance). 3/25

4 To analyse the entire surfaces behaviour during dynamic loading it is necessary to describe generally the wear obtained by two testing instruments. IMPACT TESTER Fatigue strength Contact wear FRETTING TESTER Fretting wear Wear from vibrations (oscillation) Fatigue wear 4/25

5 1) Steel with nitride layers: a) steel ASL 817 with ion nitriding surface coating sample no.1 b) steel ASL 813 with ion nitriding surface coating sample no.2 c) steel ASL 813 with gas nitriding surface coating sample no.3 2) Nodular cast iron with chrome coatings: a) galvanic porous chrome with content of Al 2 O 3 particles sample no.4 b) galvanic porous chrome without content of Al 2 O 3 particles sample no.5 Coatings and layers thicknesses [µm] Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no /25

6 6/25 Sample no.1 Surfaces of nitride layers Sample no.2 Sample no.3 Different quality of sample surfaces

7 Impact test impact energy 25 J PIN ball WC number of impacts: nitride layer : 1 000, 2 500, 5 000, Weight < 10 N Pin holder max. 65 chrome coatings : 1 000, 2 500, (due to smaller resistance) impact craters were monitored by: light microscopy, confocal microscopy, electron microscopy depth of craters was measured contactless by laser confocal microscope Olympus LEXT Pin 4 6 mm Coating Substrate 7/25

8 Impact test enables testing of selected coatings and layers for fatigue strength (while e.g. tests based on the scratch test or tribological experiments can be inadequate for condition simulation, when the surface is exposed both to fatigue and contact wear) truly simulates real situations within coatings lifetime principle lies in periodic frequency impact under certain loaded PIN (e.g. Al 2 O 3, tungsten carbide and ČSN steel) impact tester is equipped with blow apparatus damaged and removed wear debris will not remain in the contact area and results are not influenced 8/25

9 is caused by the oscillating movement with small amplitude that may occur between contacting surfaces subjected to vibration is a special kind of fatigue surface wear is a dynamic process which is strongly effected by vibration, contact area, tribochemical influence and wear debris play an important role there direct output is course of friction coefficient in dependence on number of cycles character and wear of PIN ball and wear track are observed Fretting test 9/25

10 Fretting test PIN - ČSN 17042, 1000 cycles for exact measuring of friction coefficient PIN WC, 5000 and cycles to monitor resistance of each coatings and layers to wear Track length of 1 cycle µm 10/25

11 Nitride layers

12 Number of impacts Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no.5 Depth of crater [µm] 9,71 13,77 15,9 22,5 27,75 Width of crater [µm] 644,9 683,65 717,35 789,8 911,2 Depth of crater [µm] 14,39 15,49 20,9 27,25 65 Width of crater [µm] 716,35 712,25 724,45 766,3 1420,45 Depth of crater [µm] 15,78 16,04 24, Width of crater [µm] 721,45 704,1 730,6 921, Depth of crater [µm] 19,51 18,36 28, Width of crater [µm] ,8 781, Depth [μm] ,0 65,0 31,0 27,3 27,8 22, Number of impacts with Al2O3 without Al2O3 Depths of impact craters of chrome coatings width - depth rate ,49 40,29 27,19 45,7 43,89 30,04 49,79 45,98 34,67 66,41 49,64 45, number of impacts Sample no.1 Sample no.2 Sample no.3 Strengthening processes which take place in contact area are evident from width - depth rate of impact crater Dependences of width - depth rate of impact crater on number of impact of tested nitride layers Depth increase in dependence on number of impacts is linear low resistance to impact straining 12/25

13 The gradual growth of the crater illustrates, that the strengthening process does not take place on its surface in the contact area. Strengthening processes could stop or slow down the linear growth of impact depth, as in the case of samples no.1 and 2. Depth [μm] ,51 18,36 28,75 15,78 16,04 24,32 14,39 15,49 20,90 9,71 13,77 15, Number of impacts Sample no. 1 Sample no. 2 Sample no. 3 Sample no.1 had the best resistance to low-cycle dynamic straining. Sample no.3 showed low resistance to low and high-cycle dynamic straining. 13/25

14 70 66,41 Width - depth rate width - depth rate ,49 40,29 27,19 45,7 43,89 30,04 49,79 45,98 34,67 49,64 45,13 Sample no.1 Sample no.2 Sample no number of impacts Sample no.2 small difference of width - depth rate of impact crater between and impacts gradual increasing of crater width without increasing of impact depth Intensive straining The highest resistance to the repetitive dynamic impact straining of sample no.2 14/25

15 Sample no.3 after 5000 impacts Sample no.1 after impacts edge of impacts Adhesive failure of sample no.2 in the edge of impacts. Edge of impacts = transition of tensile stress to compressive contrarious stress condition Low toughness of layers 15/25

16 Fretting test Sample Sample no.1 Sample no.2 Sample no.3 Number of cycles Load [N] PIN Width of track [µm] Wear of track [mm 3 ] Average depth of track [µm] ČSN ,0002 0, WC 171 0,0008 1, WC 347 0,0017 1, ČSN ,0002 0, WC 182 0,0004 0, WC 217 0,0005 0, ČSN ,0002 0, WC 221 0,0006 0, WC 271 0,0009 0,8 sample no.1 exhibited the highest wear of all tested nitride layers when the PIN counterpart from tungsten carbide was used (due to low hardness) sample no.2 exhibited the best wear resistance 16/25

17 friction coefficient 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, number of cycles sample no.1 sample no. 2 sample no.3 Course of friction coefficient of samples 1, 2, 3, F= 5 N, n = cycles, PIN WC. the lowest friction coefficient ( 0,4) was measured in sample no.2 with PIN from tungsten carbide. the highest friction coefficient which was evaluated in case of sample no.3 ( 0,63). course of friction coefficient changed with the use of steel PIN. The lowest friction coefficient was measured in sample no.3, in comparison with sample no.2 exhibiting the highest one. Sample no.1, 5000 cycles, 5N, WC PIN Sample no.2, 5000 cycles, 5N, WC PIN Sample no.3, 5000 cycles, 5N, WC PIN

18 Hardnesses and Microhardnesses Portion of elastic and total deformation 100 Value [%] mn 20mN 30mN 50mN 70mN 100 mn 200mN Load [mn] 300mN 500mN 700mN 1000mN Sample 1 Sample 2 Sample 3 Sample no.3 had considerably higher elastic deformation ability (lower ability of plastic deformation) Sample no.1 Sample no.2 Sample no.3 Sample no.4 Sample no.5 HV0,02 [GPa] 11,92±0,7 11,48±0,32 13,80±0,46 11,97±0,93 12,02±0,46 HV5 [GPa] 7,998±0,390 6,500±0,278 10,21±1,624 6,232±0,297 6,042±0,333 all analysed samples reach similar values of microhardness hardness values were quite different at HV5 load with respect to contact stress, the indentations at F=294,3N were carried out - initiation of surface cracks round about the imprints was detected 18/25

19 to evaluate both the cracks and imprints the laser confocal microscope was used sample no. 3 shows the highest initiation of damage to the wear neighbourhood where the imprints were carried out it's evidently evoked by its high hardness, when minimum of strain is eliminated by plastic deformation and elastic deformation is insufficient to eliminate of expand the cracks Portion of elastic and total deformation 100 Value [%] mn 20mN 30mN 50mN 70mN 100 mn 200mN Load [mn] 300mN 500mN 700mN 1000mN Sample 1 Sample 2 Sample 3 19/25

20 Chrome coatings

21 1 000 impacts Chrome coatings with content of Al 2 O 3 Chrome coatings without content of Al 2 O impacts 21/25

22 Depth [μm] ,0 65,0 31,0 Depths of impact craters 27,3 27,8 22, Number of impacts with Al2O3 without Al2O3 The poruses significantly influenced coating resistance. Poruses have ability to eliminate accumulated tensile stress effects. X Al 2 O 3 particles and their distribution can negatively affect cohesion of the coating. Coating without Al 2 O 3 particles exhibited the best wear resistance to impact straining. 22/25

23 Decohesion of chrome coating with Al 2 O 3 particles Impacts craters after impacts. Gradual breaking off of the coating creation of the graded structure Influence of surface roughness increase on impact wear resistance was not proved. 23/25

24 firction coefficient 0,6 0,5 0,4 0,3 0,2 0, Coating w ith Al2O3 Coating w ithout Al2O3 Chrome coatings Chrome coating with Al 2 O 3 particles number of cycles due to of unequal surface topography in the case of chrome sample with Al 2 O 3 particles, original surface layer remained in the wear track wear debris consisted of material from layer and the PIN counterpart coating with Al 2 O 3 particles was more intensively worn; delamination of this coating was not observed, the layer was only deformed or cohesive failure occurred. surface topography of coating without Al 2 O 3 particles was markedly higher than layer with Al 2 O 3 particles a larger part of original surface preserved. accumulation of wear debris in the wear track and delamination of layers were not observed plastic deformation was detected, which evoked cohesive failure of layers Chrome coating without Al 2 O 3 particles 24/25

25 Conclusions The conformity between results from the fretting test and impact test were found. The best resistance to impact and fatigue failure was exhibited by the nitride layers sample no. 2 - steel ASL 813 with ion nitriding surface coating In chrome coatings the best wear resistance was monitored in porous chrome without the content of Al 2 O 3 particles. Results of the microhardness measurement of nitride layers and coatings would not be sufficient for the explanation of fatigue behaviour and application possibilities of investigated samples it is not possible from the microhardness measurement alone to predict the resistance of layers and coatings in industry applications. 25/25

26 Thank you for your attention