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Piston ring coatings for high horsepower diesel engines
Surface and Coatings Technology, 61(1993) 36-42
36
Piston ring coatings for high horsepower diesel engines F. Rastegar and A. E. Craft Cummins Engine Company, Piston Ring Division, 4500 Leeds Avenue, Suite 118, Charleston, SC 29405 (USA)
Abstract Thermal-spray-deposited face coatings were developed for a top compression piston ring operating under the high pressures and temperatures of high horsepower diesel engines. Coatings were deposited by plasma spray and high velocity oxygen fuel (HVOF) techniques onto the piston ring faces. The coatings were evaluated by wear and engine tests. Three coatings were deposited by a plasma spray technique: molybdenum/chromium carbide, molybdenum/molybdenum carbide, and chromium oxide. Nickel chromium/chromium carbide powder was deposited by a HVOF technique. On the basis ofwear and engine testing, HVOF nickel chromium/chromium carbide, plasma-sprayed chromium oxide and plasmasprayed molybdenum/chromium carbide were identified to have wear resistances superior to that of today’s electroplated chromium and molybdenum carbide coatings. Cylinder liner wear was found to be generally equivalent or lower for the thermal spray coatings when compared with the electroplated chromium.
1. Introduction Strict environmental emission laws have forced major changes in the design of power cylinder components for currently produced diesel engines. Lubricating oil consumed during engine operation contributes to one third of the particulates emitted. Therefore it becomes essential to reduce the oil exposure to combustion gases by elevating the top compression ring closer to the top of the piston. Raising the compression piston ring position will force the ring to operate under increasingly unfavorable conditions: higher combustion forces, elevated ternperatures and thinner lubricating films. The face coating is the major feature of the piston ring which is affected. The majority of top compression rings produced today are plated with chromium. Chromium plating has proven to have poor wear and scuff-resistant properties at higher ring temperature and boundary lubrication conditions. Thermal spray coatings have been used as substitutes for chromium plating for more than two decades. The face coatings used are typically molybdenum based with hard phase constituents such as chromium carbide [I 2]. Even though the chromium-plated piston rings are effective in today’s engines, they are the limiting factor in engine endurance [1]. For sealing purposes it is desirable to have highly wear-resistant ring face coatings. The ring gap area increases as the top compression ring face coatings wear, thereby allowing blow-by of combustion gases to the sump. Liner wear also causes a ring gap area increase; therefore it is essential to minimize induced wear caused by the ring face. Consequently, compatible ring and liner materials allowing for lower
0257—8972/93/$6.OO
system wear and component longevity have become essential. Initially, using test samples, wear-resistant thermal spray coatings are identified by employing bench test techniques which simulate the reciprocating motion of a piston ring against the cylinder liner [3, 4]. Test coatings are ranked on the basis of coating wear resistance and liner wear compared with today’s chromium electroplated coatings. The selected thermal spray coatings are then deposited on piston rings and engine tested under steady state conditions. After successfully passing steady state engine tests, the coated piston rings are then tested under more severe engine conditions and eventually introduced as a product. A series of face coatings were developed on the basis of the criteria and testing techniques described. The following coatings were selected to be plasma sprayed: molybdenum/chromium carbide, molybdenum/molybdenum carbide and chromium oxide. Nickel chromium/ chromium carbide powder was deposited by the high velocity oxygen fuel (HVOF) technique. Molybdenum is traditionally used in plasma-sprayed piston ring coatings to enhance scuff resistance and to improve coating cohesion.
2. Experimental procedures 2.1. Material selection and spray processes Rectangular ductile iron piston ring blanks were machined such that the faces contained a groove. The piston ring surfaces were then vapor degreased and grit
©
1993
—
Elsevier Sequoia. All rights reserved
F. Rastegar, A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
blasted with a 60 grit A1203 medium. A typical piston ring blank geometry is shown in Fig. 1. Wear coatings were then applied to the ring faces by either plasma spraying or HYOF techniques. After plasma deposition, the coatings were ground to a surface finish Ra of 0.41—0.64 ~im (16—25 jtin) and then lapped to form a crown with a final surface finish Ra of 0.36—0.46 l.tm (14—18 ~tin).In the case of the HVOF samples, a square blank was used and a final surface finish Ra of 0.08—0.15 j.tm (3—6 pin) was produced. 2.2. Starting powders and deposition techniques 2.2.1. Molybdenum/chromium carbide A proprietary chromium carbide powder was prealloyed with nickel chromium. The nickel chromium/ chromium carbide alloy was then blended with spherical molybdenum powder and plasma sprayed. 2.2.2. Molybdenum/molybdenum carbide Molybdenum and molybdenum carbide particles were sintered together with a binder to produce an aggregate powder. This aggregate was a composite of 30 wt.% Mo2C particles with the remainder being molybdenum. This powder was mechanically blended with a nickel chromium powder to improve the cohesion and adhesion of the coating. The molybdenum/molybdenum carbide coating was applied using plasma spraying. 2.2.3. Chromium oxide and bond coat Chromium oxide powder (98% pure) was selected for this development [5, 6]. A nickel, chromium, aluminum and yttrium (Ni—Cr--Al--Y) powder was used to produce a bond coat to improve coating adhesion. Finally, the chromium oxide top coat was deposited, with all powder samples coming from one lot. The bond and top coats were deposited using plasma spraying. 2.2.4. Nickel chromium/chromium carbide The nickel chromium/chromium carbide alloyed powder particles were made from nickel, chromium and
37
chromium carbide and had an angular shape. This powder was applied by HVOF using a Metco Diamond Jet gun and its supporting system. 2.3. Spray processes All plasma coatings were applied with a Metco 7MB gun. Argon was used as the primary plasma gas with hydrogen as the secondary gas. A constant-current plasma arc was employed to maintain a pre-selected power. Deposition conditions were optimized to provide the highest hardness and lowest porosity for all coatings. The primary gas flow rates were changed to vary the plume enthalpy [5, 6]. A Metco HVOF Diamond Jet system was used to deposit the chromium carbide coating onto the ductile iron piston rings. Special cooling arrangements had to be devised to avoid overheating the piston rings (an inherent problem of this technique). The substrate ternperature was held at under 93 °C (200 °F)during the coating process. 2.4. General testing techniques The starting powders were analyzed for metallic chemistry using energy-dispersive spectroscopy (EDS). In the case of chromium carbide applied by HVOF, the carbon chemistry was also obtained. The powders were sieved to calculate the particle size distribution, and powder morphologies were further examined by scanning electron microscopy (SEM). Limited use of X-ray diffraction (XRD) was employed to discern the phase contents of some of the starting powders. Piston ring coating samples were prepared using standard metallographic techniques to evaluate a number of properties, including the hardness, the oxide formation, the percentage porosity and the amount of unmelted particles. The coating hardness was evaluated using a Vickers microhardness tester with a diamond pyramid indenter set for a 300 gf load. The other properties listed above were evaluated by visual examination of photomicrographs. In addition, all coating metallic chemistries were measured using EDS, and some coating phase contents were evaluated by XRD. A twist test was also employed to determine the relative adhesion of each coating [5, 6]. In this test, one ring tip was held stationary while the opposite tip was twisted in a plane perpendicular to the ring until the coating debonded from the substrate. 2.5. Wear testing
Fig. 1. Schematic diagram of the piston ring geometry in cross-section.
The Cameron Plint wear tester was used in bench testing because it simulates a reciprocating sliding motion [3, 5, 6]. A load was applied normal to the moving sample, in this case a ring segment. The speed of the ring movement was controlled by changes in the
38
F. Rastegar, A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
frequency of a motor. The normal force created a line contact between the ring and the liner. The liner material was heated from below and heat was conducted to the ring segment through the line contact. The lubricant was supplied externally to the piston ring and liner contact line. The specific test parameters are listed in Table 1. Volumetric wear of face coatings was measured from maximum diametrical wear. Liner wear was concluded from surface traces, and the total volume removed was calculated. All values were then divided by the sliding distance and applied normal force. The final product is a wear rate coefficient which is independent of ring diameter and crown radius.
TABLE 2. Coating properties measured Coating
Coating properties .
Composition (wt. %)
Mo/Cr 3C2 Mo/M02C
Bulk Porosity hardness (%) (HV 300 gf)
Proprietary Mo2C, 25; Ni-Cr 20’ Mo, balance
Cr203 (NiCr)/Cr3C2
Cr, 98; Ti, trace Cr3C~80;
>
Failure angle (°)
660
5
453
<5
70 70
<5
70
45 <5
65
l225~ l400i~ 745
________________________________________________
aLow enthalpy plasma. bHigh enthalpy plasma.
2.6. Engine testing All coatings were tested in a six-cylinder Cummins engine operating at 560 kW (750 hp). Coated piston rings were engine tested under steady state and high thermal load conditions, The objective of the steady state engine test was to establish ring and liner wear rates and were conducted by over-fueling and over-speeding the engine. Standard plateau honed cylinder liners were used in contact with all the coatings. Ring crowns and cylinder liner dimensions were measured before and after steady state engine tests, and average wear values were calculated. The ring wear profiles were measured at 180°from the gap. The most promising coating found by the steady state test was then subjected to a high thermal load engine test. The scuff resistance of the face coating was determined by this high thermal load test. This test was performed by increasing the intake manifold and coolant temperatures. The engine was also operated at overfueled and over-speed conditions,
3. Results and discussion The coating properties listed in Table 2 are discussed in more detail in the sections following,
TABLE 1. Bench wear-testing parameters Lubrication Lubrication flow rate Temperature Stroke distance Stroke frequency Samples Test duration
SAE 30 Fully flooded 200CC 4 mm 35 Hz Piston ring segment Honed cylinder liner 6h
3.1. Molybdenum/chromium carbide plasma coating results The average bulk microhardness of the plasmasprayed molybdenum/chromium carbide coating is 660 HV 300 gf, with hardnesses ranging from 507 to 821 HV 300 gf, and the coating porosity is 5%. 3.2. Molybdenum/molybdenum carbide plasma coating results The average bulk hardness of the molybdenum/molybdenum carbide coating is 453 HV 300 gf, with a coating porosity of less than 5%. SEM examination of this coating shows distinct multiphase regions, as seen in the backscattered electron micrograph in Fig. 2(a) and the elemental map in Fig. 2(b). The phase boundaries between the nickel chromium and the molybdenum/molybdenum carbide phases appear to be free of oxidation and porosity. SEM elemental maps show a uniform distribution of nickel and chromium. 3.3. Chromium oxide plasma coating results Plasma enthalpy has a direct effect on the chromium oxide coating hardness. Higher enthalpy plasmas produce harder denser ceramic coatings. Cross-sectional average hardness ranges from 1225 HV 300 gf for the low enthalpy plasma to greater than 1400 HV 300 gf for the high enthalpy plasma [5]. The coating porosity is 5%, as seen in Fig. 3. No visible unmelted chromium oxide is evident in the cross-section. Initial coating break-out occurred at a 38°twist angle which was later improved to 70°by using a bond coat. Further details of the chromium oxide testing and results are available in refs. 5 and 6. 3.4. Nickel chromium/chromium carbide high velocity oxygen fuel coating results The average bulk hardness of the nickel chrom.
.
.
.
ium/chromium carbide coating is 745 HV 300 gf. The
F.
Rastegar,
A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
39
plated ring. The HVOF-sprayed nickel chromium/
-
chromium carbide coating has a significantly lower ring face wear rate coefficient than the chromium plating, and lower cylinder liner wear. The plasmasprayed molybdenum/chromium carbide coating and molybdenum/molybdenum carbide coatings both have lower ring face wear rate coefficients than the chromium plating, and liner wear rate coefficients equivalent to that of the chromium plating.
~— _____
___________
.4
_________________
(a) ____________
______
r~
_____
~J ~ ~ Original Image ___________
Cr
‘
IM
Ib) Fig. 2. (a) SEM backscattered electron image showing phase regions of molybdenum and molybdenum carbide (light regions) and nickel chromium (dark regions) in the molybdenum/molybdenum carbide plasma-sprayed coating cross-section. (b) SEM elemental map of the molybdenum/molybdenum carbide plasma coating cross-section. The maps show (clockwise from upper left) molybdenum, nickel, overall image and chromium, each as light regions.
adhesion failure angle was 65°for the twist test. Porosity is well below 5% and there is no significant evidence of unmelted particles, XRD patterns were made for both the nickel chromiurn/chromium carbide powder and HVOF-sprayed coating (Fig. 4). From XRD analysis, this coating seems highly amorphous (largely non-crystalline), giving low intensity diffraction peaks. 3.5. Wear test results The wear test results are summarized in Fig. 5. The total ring and cylinder liner wear for the plasma-sprayed chromium oxide coating is the lowest of all tested coatings. The chromium oxide ring has a much lower ring face wear rate coefficient and a significantly lower cylinder liner wear rate coefficient than the chromium-
3.6. Steady state engine test results A series of chromium-plated piston rings were engine tested to establish a baseline wear rate for rings and cylinder liners. Cylinder liner and piston ring wear data for chromium-plated and thermally sprayed rings are ~ suffered severe abrasion wear (Fig. 7) with nearly 75% of the face coating worn away after the 250 h test. Plasma-sprayed molybdenum/molybdenum carbide, molybdenum/chromium carbide and HVOF-sprayed nickel chromium/chromium carbide coatings were engine tested for a period of 250 h under steady state conditions. The plasma-sprayed chromium oxide rings have currently had limited engine testing and wear data are no avai a e. The plasma-sprayed molybdenum/molybdenum carbide face coating has an equivalent wear rate to the chromium plating. However, there is significantly lower cylinder liner wear with the molybdenum/molybdenum carbide, equalling nearly one half that of the chromium plating induced liner wear. The plasma-sprayed molybdenum/chromium carbide .
.
face coating wear is one third of the face coating wear of electroplated chromium but has equivalent cylinder liner wear. A photograph of the molybdenum/chromium carbide coating after engine testing is presented in Fig. 8. This coating has proven to be highly wear and abrasion resistant. The HVOF-sprayed nickel chromium/chromium carbide coating has significantly lower face wear than the rest of the ring coatings tested. The nickel chromium/ chromium carbide coating face wear is one tenth of that of the electroplated chromium, with equivalent bore wear. Only moderate abrasion signs can be seen on the nickel chromium/chromium carbide face coating. In addition, the crown wear on the ring is extremely low and only asperity polishing of the crown occurred (Fig. 9). From these series of the engine tests, the plasmasprayed molybdenum/chromium carbide coating was selected for further engine testing under more severe conditions (high thermal load), since it has the lowest face wear.
40
F. Rastegar, A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
Fig. 3. Photomicrograph of the plasma-sprayed chromium oxide coating (dark) on an Ni—Cr—Al—Y base coat (light), in cross-section. (The dimensions are proprietary.)
2—Theta
—
C,, C”
Scale I
I
I
I
I
I
I
I
C.; ICC
a,
a,
(a)
0
2—Theta C.,
—
Scale
I
I
I
C., 0 C,,
(b)
°
~
40
~0
Fig. 4. (a) XRD pattern of the nickel chromium/chromium carbide powder showing the nickel chromium and chromium carbide phases. (b) XRD pattern of the nickel chromium/chromium carbide HVOF-sprayed coating showing diffuse peaks which indicated retained nickel chromium and chromium carbide phases after HVOF spraying. The low intensity broad peaks imply that much of the coating may be amorphous or noncrystalline.
F. Rastegar, A. E. Craft
/
I
r
~ lo~
p7
.
HVOF-Cr32 C Cr 23 0
MoCr 32 C
__ ______
41
~
~ ~IRingWear
~ V ~ V ~ ~ V ~ V ~
~,_,~~
10.12
Piston rim’ coatings for high horsepower diesel engines
~~ Liner Wear I
~ Cr Plate
Mo 2C
J
Fig. 8. The plasma-sprayed molybdenum/chromium carbide coating shows little wear damage after a 250 h steady state condition engine test. (The dimensions are proprietary.)
Fig. 5. Summary of the piston ring coating and cylinder liner wear test results for all samples compared with electroplated chromium. —
10000
-~~ilI
1
1000
‘~
i
-~
4
.
10
Fig. 9. The HVOF-sprayed nickel chromium/chromium carbide coat__________
~ Ring Wear 1
~ HvoF-Crc
MoCrC
MoC
ing provides evidence of only moderate wear damage after steady state condition engine testing for 250 h. (The dimensions are proprietary.)
Liner Wear
Cr Plate
Fig. 6. Summary of the piston ring coating and cylinder liner wear results under steady state condition engine tests comparing plasmaand HYOF-sprayed coatings and electroplated chromium.
~
Fig. 10. Severe scuffing and full-face adhesive wear is shown for chromium-plated piston rings after high thermal load engine testing for 200 h. (The dimensions are proprietary.)
1.)UJIL
~
1
J.jer
Fig. 7. Abrasion wear damage is evident on the face of a chromiumplated piston ring after a 250 h steady state condition engine test. Nearly 75% of the coating face was worn away. (The dimensions are proprietary.)
3.7. High thermal load engine test results
The plasma-sprayed molybdenum/chromium carbide coating was selected for a final thermal test. An engine was assembled with three chromium-plated and three plasma-sprayed molybdenum/chromium carbide piston rings and was operated under a cyclic test condition, The chromium plating suffered severe adhesive wear (incipient scuffing) with full face wear occurring after
—
_____
‘~1IL Fig. 11. After a high thermal load engine test (200 h), the plasmasprayed molybdenum/chromium carbide coating demonstrates low abrasive wear damage and high scuff resistance. (The dimensions are proprietary.)
200 h (Fig. 10). In contrast, the plasma-sprayed molybdenum/chromium carbide coating had significantly lower wear than the chromium plating and had high scuff resistance (Fig. 11).
42
F. Rastegar, A. F. Craft
/
Piston ring coatings for high horsepower diesel engines
4. Conclusions HVOF-sprayed nickel chromium/chromium carbide, plasma-sprayed molybdenum/chromium carbide and plasma-sprayed chromium oxide piston ring face coatings proved to have superior wear resistance over both the plasma-sprayed molybdenum/molybdenum carbide and the electroplated chromium. The cylinder liner wear induced by each of the thermal spray coatings was equivalent to or less than that of today’s chromium
plating.
Gray for their patience, dedication and hard work during the course of this coating development program. This work would not have been possible without the great help of all personnel at Cummins Piston Ring Division.
References 1 W. B. Young and J. A. McComb, New piston ring face coatings using design of Experiments, SAE Paper 900588, 1990, Society of Automotive Engineers, New York. 2 G. F. Hyde, J. E. Cromwell and J. H. Barnes, Piston ring coatings
The plasma-sprayed molybdenum/chromium carbide coating shows great promise as an alternative to chromium plating in high horsepower diesel engines. In addition, HVOF-sprayed nickel chromium/chromium carbide and plasma-sprayed chromium oxide coatings show excellent potential for use in high horsepower diesel engines; however, more process and materials development and testing are needed.
for internal combustion engines, SAE Paper 790865, 1979, Society of Automotive Engineers, New York. 3 M. G. S. Naylor and M. P. Fear, Development of wear resistant ceramic coatings for in-cylinder diesel engine components, Proc. Coatings for advanced heat engines Workshop, Washington, DC, 1990, US Department of Energy, Washington, DC, 1990, pp. III93—111-102. 4 H. Yoshida, K. Kusama and J. Sagawa, Effects of surface treatments on piston ring friction force and wear, SAE Paper 900589, 1990, Society of Automotive Engineers, New York. 5 F. Rastegar, Ring coatings for a low heat rejection engine, in C. C. Berndt, (ed), Proceedings of the International Thermal Spray Conf.
Acknowledgments
and Exposition, American Society for Metals, Materials Park, OH, 1992, pp. 743—748. 6 F. Rastegar and D. E. Richardson, Ring development for low heat rejection diesel engines, Proc. Coatings for Advanced Heat Engines
The authors would like to thank Mr. Pierz for managing the engine testing program. In addition, the authors would like to thank Mr. David Boone and Mr. Jerry
Workshop, Washington, DC, 1992, US Department of Energy, Washington, DC, 1992, in press.
36
Piston ring coatings for high horsepower diesel engines F. Rastegar and A. E. Craft Cummins Engine Company, Piston Ring Division, 4500 Leeds Avenue, Suite 118, Charleston, SC 29405 (USA)
Abstract Thermal-spray-deposited face coatings were developed for a top compression piston ring operating under the high pressures and temperatures of high horsepower diesel engines. Coatings were deposited by plasma spray and high velocity oxygen fuel (HVOF) techniques onto the piston ring faces. The coatings were evaluated by wear and engine tests. Three coatings were deposited by a plasma spray technique: molybdenum/chromium carbide, molybdenum/molybdenum carbide, and chromium oxide. Nickel chromium/chromium carbide powder was deposited by a HVOF technique. On the basis ofwear and engine testing, HVOF nickel chromium/chromium carbide, plasma-sprayed chromium oxide and plasmasprayed molybdenum/chromium carbide were identified to have wear resistances superior to that of today’s electroplated chromium and molybdenum carbide coatings. Cylinder liner wear was found to be generally equivalent or lower for the thermal spray coatings when compared with the electroplated chromium.
1. Introduction Strict environmental emission laws have forced major changes in the design of power cylinder components for currently produced diesel engines. Lubricating oil consumed during engine operation contributes to one third of the particulates emitted. Therefore it becomes essential to reduce the oil exposure to combustion gases by elevating the top compression ring closer to the top of the piston. Raising the compression piston ring position will force the ring to operate under increasingly unfavorable conditions: higher combustion forces, elevated ternperatures and thinner lubricating films. The face coating is the major feature of the piston ring which is affected. The majority of top compression rings produced today are plated with chromium. Chromium plating has proven to have poor wear and scuff-resistant properties at higher ring temperature and boundary lubrication conditions. Thermal spray coatings have been used as substitutes for chromium plating for more than two decades. The face coatings used are typically molybdenum based with hard phase constituents such as chromium carbide [I 2]. Even though the chromium-plated piston rings are effective in today’s engines, they are the limiting factor in engine endurance [1]. For sealing purposes it is desirable to have highly wear-resistant ring face coatings. The ring gap area increases as the top compression ring face coatings wear, thereby allowing blow-by of combustion gases to the sump. Liner wear also causes a ring gap area increase; therefore it is essential to minimize induced wear caused by the ring face. Consequently, compatible ring and liner materials allowing for lower
0257—8972/93/$6.OO
system wear and component longevity have become essential. Initially, using test samples, wear-resistant thermal spray coatings are identified by employing bench test techniques which simulate the reciprocating motion of a piston ring against the cylinder liner [3, 4]. Test coatings are ranked on the basis of coating wear resistance and liner wear compared with today’s chromium electroplated coatings. The selected thermal spray coatings are then deposited on piston rings and engine tested under steady state conditions. After successfully passing steady state engine tests, the coated piston rings are then tested under more severe engine conditions and eventually introduced as a product. A series of face coatings were developed on the basis of the criteria and testing techniques described. The following coatings were selected to be plasma sprayed: molybdenum/chromium carbide, molybdenum/molybdenum carbide and chromium oxide. Nickel chromium/ chromium carbide powder was deposited by the high velocity oxygen fuel (HVOF) technique. Molybdenum is traditionally used in plasma-sprayed piston ring coatings to enhance scuff resistance and to improve coating cohesion.
2. Experimental procedures 2.1. Material selection and spray processes Rectangular ductile iron piston ring blanks were machined such that the faces contained a groove. The piston ring surfaces were then vapor degreased and grit
©
1993
—
Elsevier Sequoia. All rights reserved
F. Rastegar, A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
blasted with a 60 grit A1203 medium. A typical piston ring blank geometry is shown in Fig. 1. Wear coatings were then applied to the ring faces by either plasma spraying or HYOF techniques. After plasma deposition, the coatings were ground to a surface finish Ra of 0.41—0.64 ~im (16—25 jtin) and then lapped to form a crown with a final surface finish Ra of 0.36—0.46 l.tm (14—18 ~tin).In the case of the HVOF samples, a square blank was used and a final surface finish Ra of 0.08—0.15 j.tm (3—6 pin) was produced. 2.2. Starting powders and deposition techniques 2.2.1. Molybdenum/chromium carbide A proprietary chromium carbide powder was prealloyed with nickel chromium. The nickel chromium/ chromium carbide alloy was then blended with spherical molybdenum powder and plasma sprayed. 2.2.2. Molybdenum/molybdenum carbide Molybdenum and molybdenum carbide particles were sintered together with a binder to produce an aggregate powder. This aggregate was a composite of 30 wt.% Mo2C particles with the remainder being molybdenum. This powder was mechanically blended with a nickel chromium powder to improve the cohesion and adhesion of the coating. The molybdenum/molybdenum carbide coating was applied using plasma spraying. 2.2.3. Chromium oxide and bond coat Chromium oxide powder (98% pure) was selected for this development [5, 6]. A nickel, chromium, aluminum and yttrium (Ni—Cr--Al--Y) powder was used to produce a bond coat to improve coating adhesion. Finally, the chromium oxide top coat was deposited, with all powder samples coming from one lot. The bond and top coats were deposited using plasma spraying. 2.2.4. Nickel chromium/chromium carbide The nickel chromium/chromium carbide alloyed powder particles were made from nickel, chromium and
37
chromium carbide and had an angular shape. This powder was applied by HVOF using a Metco Diamond Jet gun and its supporting system. 2.3. Spray processes All plasma coatings were applied with a Metco 7MB gun. Argon was used as the primary plasma gas with hydrogen as the secondary gas. A constant-current plasma arc was employed to maintain a pre-selected power. Deposition conditions were optimized to provide the highest hardness and lowest porosity for all coatings. The primary gas flow rates were changed to vary the plume enthalpy [5, 6]. A Metco HVOF Diamond Jet system was used to deposit the chromium carbide coating onto the ductile iron piston rings. Special cooling arrangements had to be devised to avoid overheating the piston rings (an inherent problem of this technique). The substrate ternperature was held at under 93 °C (200 °F)during the coating process. 2.4. General testing techniques The starting powders were analyzed for metallic chemistry using energy-dispersive spectroscopy (EDS). In the case of chromium carbide applied by HVOF, the carbon chemistry was also obtained. The powders were sieved to calculate the particle size distribution, and powder morphologies were further examined by scanning electron microscopy (SEM). Limited use of X-ray diffraction (XRD) was employed to discern the phase contents of some of the starting powders. Piston ring coating samples were prepared using standard metallographic techniques to evaluate a number of properties, including the hardness, the oxide formation, the percentage porosity and the amount of unmelted particles. The coating hardness was evaluated using a Vickers microhardness tester with a diamond pyramid indenter set for a 300 gf load. The other properties listed above were evaluated by visual examination of photomicrographs. In addition, all coating metallic chemistries were measured using EDS, and some coating phase contents were evaluated by XRD. A twist test was also employed to determine the relative adhesion of each coating [5, 6]. In this test, one ring tip was held stationary while the opposite tip was twisted in a plane perpendicular to the ring until the coating debonded from the substrate. 2.5. Wear testing
Fig. 1. Schematic diagram of the piston ring geometry in cross-section.
The Cameron Plint wear tester was used in bench testing because it simulates a reciprocating sliding motion [3, 5, 6]. A load was applied normal to the moving sample, in this case a ring segment. The speed of the ring movement was controlled by changes in the
38
F. Rastegar, A. E. Craft
/
Piston ring coatings for high horsepower diesel engines
frequency of a motor. The normal force created a line contact between the ring and the liner. The liner material was heated from below and heat was conducted to the ring segment through the line contact. The lubricant was supplied externally to the piston ring and liner contact line. The specific test parameters are listed in Table 1. Volumetric wear of face coatings was measured from maximum diametrical wear. Liner wear was concluded from surface traces, and the total volume removed was calculated. All values were then divided by the sliding distance and applied normal force. The final product is a wear rate coefficient which is independent of ring diameter and crown radius.
TABLE 2. Coating properties measured Coating
Coating properties .
Composition (wt. %)
Mo/Cr 3C2 Mo/M02C
Bulk Porosity hardness (%) (HV 300 gf)
Proprietary Mo2C, 25; Ni-Cr 20’ Mo, balance
Cr203 (NiCr)/Cr3C2
Cr, 98; Ti, trace Cr3C~80;
>
Failure angle (°)
660
5
453
<5
70 70
<5
70
45 <5
65
l225~ l400i~ 745
________________________________________________
aLow enthalpy plasma. bHigh enthalpy plasma.
2.6. Engine testing All coatings were tested in a six-cylinder Cummins engine operating at 560 kW (750 hp). Coated piston rings were engine tested under steady state and high thermal load conditions, The objective of the steady state engine test was to establish ring and liner wear rates and were conducted by over-fueling and over-speeding the engine. Standard plateau honed cylinder liners were used in contact with all the coatings. Ring crowns and cylinder liner dimensions were measured before and after steady state engine tests, and average wear values were calculated. The ring wear profiles were measured at 180°from the gap. The most promising coating found by the steady state test was then subjected to a high thermal load engine test. The scuff resistance of the face coating was determined by this high thermal load test. This test was performed by increasing the intake manifold and coolant temperatures. The engine was also operated at overfueled and over-speed conditions,
3. Results and discussion The coating properties listed in Table 2 are discussed in more detail in the sections following,
TABLE 1. Bench wear-testing parameters Lubrication Lubrication flow rate Temperature Stroke distance Stroke frequency Samples Test duration
SAE 30 Fully flooded 200CC 4 mm 35 Hz Piston ring segment Honed cylinder liner 6h
3.1. Molybdenum/chromium carbide plasma coating results The average bulk microhardness of the plasmasprayed molybdenum/chromium carbide coating is 660 HV 300 gf, with hardnesses ranging from 507 to 821 HV 300 gf, and the coating porosity is 5%. 3.2. Molybdenum/molybdenum carbide plasma coating results The average bulk hardness of the molybdenum/molybdenum carbide coating is 453 HV 300 gf, with a coating porosity of less than 5%. SEM examination of this coating shows distinct multiphase regions, as seen in the backscattered electron micrograph in Fig. 2(a) and the elemental map in Fig. 2(b). The phase boundaries between the nickel chromium and the molybdenum/molybdenum carbide phases appear to be free of oxidation and porosity. SEM elemental maps show a uniform distribution of nickel and chromium. 3.3. Chromium oxide plasma coating results Plasma enthalpy has a direct effect on the chromium oxide coating hardness. Higher enthalpy plasmas produce harder denser ceramic coatings. Cross-sectional average hardness ranges from 1225 HV 300 gf for the low enthalpy plasma to greater than 1400 HV 300 gf for the high enthalpy plasma [5]. The coating porosity is 5%, as seen in Fig. 3. No visible unmelted chromium oxide is evident in the cross-section. Initial coating break-out occurred at a 38°twist angle which was later improved to 70°by using a bond coat. Further details of the chromium oxide testing and results are available in refs. 5 and 6. 3.4. Nickel chromium/chromium carbide high velocity oxygen fuel coating results The average bulk hardness of the nickel chrom.
.
.
.
ium/chromium carbide coating is 745 HV 300 gf. The
F.
Rastegar,
A. E. Craft
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Piston ring coatings for high horsepower diesel engines
39
plated ring. The HVOF-sprayed nickel chromium/
-
chromium carbide coating has a significantly lower ring face wear rate coefficient than the chromium plating, and lower cylinder liner wear. The plasmasprayed molybdenum/chromium carbide coating and molybdenum/molybdenum carbide coatings both have lower ring face wear rate coefficients than the chromium plating, and liner wear rate coefficients equivalent to that of the chromium plating.
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_________________
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______
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Ib) Fig. 2. (a) SEM backscattered electron image showing phase regions of molybdenum and molybdenum carbide (light regions) and nickel chromium (dark regions) in the molybdenum/molybdenum carbide plasma-sprayed coating cross-section. (b) SEM elemental map of the molybdenum/molybdenum carbide plasma coating cross-section. The maps show (clockwise from upper left) molybdenum, nickel, overall image and chromium, each as light regions.
adhesion failure angle was 65°for the twist test. Porosity is well below 5% and there is no significant evidence of unmelted particles, XRD patterns were made for both the nickel chromiurn/chromium carbide powder and HVOF-sprayed coating (Fig. 4). From XRD analysis, this coating seems highly amorphous (largely non-crystalline), giving low intensity diffraction peaks. 3.5. Wear test results The wear test results are summarized in Fig. 5. The total ring and cylinder liner wear for the plasma-sprayed chromium oxide coating is the lowest of all tested coatings. The chromium oxide ring has a much lower ring face wear rate coefficient and a significantly lower cylinder liner wear rate coefficient than the chromium-
3.6. Steady state engine test results A series of chromium-plated piston rings were engine tested to establish a baseline wear rate for rings and cylinder liners. Cylinder liner and piston ring wear data for chromium-plated and thermally sprayed rings are ~ suffered severe abrasion wear (Fig. 7) with nearly 75% of the face coating worn away after the 250 h test. Plasma-sprayed molybdenum/molybdenum carbide, molybdenum/chromium carbide and HVOF-sprayed nickel chromium/chromium carbide coatings were engine tested for a period of 250 h under steady state conditions. The plasma-sprayed chromium oxide rings have currently had limited engine testing and wear data are no avai a e. The plasma-sprayed molybdenum/molybdenum carbide face coating has an equivalent wear rate to the chromium plating. However, there is significantly lower cylinder liner wear with the molybdenum/molybdenum carbide, equalling nearly one half that of the chromium plating induced liner wear. The plasma-sprayed molybdenum/chromium carbide .
.
face coating wear is one third of the face coating wear of electroplated chromium but has equivalent cylinder liner wear. A photograph of the molybdenum/chromium carbide coating after engine testing is presented in Fig. 8. This coating has proven to be highly wear and abrasion resistant. The HVOF-sprayed nickel chromium/chromium carbide coating has significantly lower face wear than the rest of the ring coatings tested. The nickel chromium/ chromium carbide coating face wear is one tenth of that of the electroplated chromium, with equivalent bore wear. Only moderate abrasion signs can be seen on the nickel chromium/chromium carbide face coating. In addition, the crown wear on the ring is extremely low and only asperity polishing of the crown occurred (Fig. 9). From these series of the engine tests, the plasmasprayed molybdenum/chromium carbide coating was selected for further engine testing under more severe conditions (high thermal load), since it has the lowest face wear.
40
F. Rastegar, A. E. Craft
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Piston ring coatings for high horsepower diesel engines
Fig. 3. Photomicrograph of the plasma-sprayed chromium oxide coating (dark) on an Ni—Cr—Al—Y base coat (light), in cross-section. (The dimensions are proprietary.)
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Fig. 4. (a) XRD pattern of the nickel chromium/chromium carbide powder showing the nickel chromium and chromium carbide phases. (b) XRD pattern of the nickel chromium/chromium carbide HVOF-sprayed coating showing diffuse peaks which indicated retained nickel chromium and chromium carbide phases after HVOF spraying. The low intensity broad peaks imply that much of the coating may be amorphous or noncrystalline.
F. Rastegar, A. E. Craft
/
I
r
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.
HVOF-Cr32 C Cr 23 0
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41
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Piston rim’ coatings for high horsepower diesel engines
~~ Liner Wear I
~ Cr Plate
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Fig. 8. The plasma-sprayed molybdenum/chromium carbide coating shows little wear damage after a 250 h steady state condition engine test. (The dimensions are proprietary.)
Fig. 5. Summary of the piston ring coating and cylinder liner wear test results for all samples compared with electroplated chromium. —
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1
1000
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.
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Fig. 9. The HVOF-sprayed nickel chromium/chromium carbide coat__________
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ing provides evidence of only moderate wear damage after steady state condition engine testing for 250 h. (The dimensions are proprietary.)
Liner Wear
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Fig. 6. Summary of the piston ring coating and cylinder liner wear results under steady state condition engine tests comparing plasmaand HYOF-sprayed coatings and electroplated chromium.
~
Fig. 10. Severe scuffing and full-face adhesive wear is shown for chromium-plated piston rings after high thermal load engine testing for 200 h. (The dimensions are proprietary.)
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Fig. 7. Abrasion wear damage is evident on the face of a chromiumplated piston ring after a 250 h steady state condition engine test. Nearly 75% of the coating face was worn away. (The dimensions are proprietary.)
3.7. High thermal load engine test results
The plasma-sprayed molybdenum/chromium carbide coating was selected for a final thermal test. An engine was assembled with three chromium-plated and three plasma-sprayed molybdenum/chromium carbide piston rings and was operated under a cyclic test condition, The chromium plating suffered severe adhesive wear (incipient scuffing) with full face wear occurring after
—
_____
‘~1IL Fig. 11. After a high thermal load engine test (200 h), the plasmasprayed molybdenum/chromium carbide coating demonstrates low abrasive wear damage and high scuff resistance. (The dimensions are proprietary.)
200 h (Fig. 10). In contrast, the plasma-sprayed molybdenum/chromium carbide coating had significantly lower wear than the chromium plating and had high scuff resistance (Fig. 11).
42
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Piston ring coatings for high horsepower diesel engines
4. Conclusions HVOF-sprayed nickel chromium/chromium carbide, plasma-sprayed molybdenum/chromium carbide and plasma-sprayed chromium oxide piston ring face coatings proved to have superior wear resistance over both the plasma-sprayed molybdenum/molybdenum carbide and the electroplated chromium. The cylinder liner wear induced by each of the thermal spray coatings was equivalent to or less than that of today’s chromium
plating.
Gray for their patience, dedication and hard work during the course of this coating development program. This work would not have been possible without the great help of all personnel at Cummins Piston Ring Division.
References 1 W. B. Young and J. A. McComb, New piston ring face coatings using design of Experiments, SAE Paper 900588, 1990, Society of Automotive Engineers, New York. 2 G. F. Hyde, J. E. Cromwell and J. H. Barnes, Piston ring coatings
The plasma-sprayed molybdenum/chromium carbide coating shows great promise as an alternative to chromium plating in high horsepower diesel engines. In addition, HVOF-sprayed nickel chromium/chromium carbide and plasma-sprayed chromium oxide coatings show excellent potential for use in high horsepower diesel engines; however, more process and materials development and testing are needed.
for internal combustion engines, SAE Paper 790865, 1979, Society of Automotive Engineers, New York. 3 M. G. S. Naylor and M. P. Fear, Development of wear resistant ceramic coatings for in-cylinder diesel engine components, Proc. Coatings for advanced heat engines Workshop, Washington, DC, 1990, US Department of Energy, Washington, DC, 1990, pp. III93—111-102. 4 H. Yoshida, K. Kusama and J. Sagawa, Effects of surface treatments on piston ring friction force and wear, SAE Paper 900589, 1990, Society of Automotive Engineers, New York. 5 F. Rastegar, Ring coatings for a low heat rejection engine, in C. C. Berndt, (ed), Proceedings of the International Thermal Spray Conf.
Acknowledgments
and Exposition, American Society for Metals, Materials Park, OH, 1992, pp. 743—748. 6 F. Rastegar and D. E. Richardson, Ring development for low heat rejection diesel engines, Proc. Coatings for Advanced Heat Engines
The authors would like to thank Mr. Pierz for managing the engine testing program. In addition, the authors would like to thank Mr. David Boone and Mr. Jerry
Workshop, Washington, DC, 1992, US Department of Energy, Washington, DC, 1992, in press.