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A composite brush-plated coating with application in the die industries
A Composite Brush-Plated Coating with Application in the Die Industries by Weijian Lin, Jingxin Chen, and Jiazhen Chen, Industrial Engineering, Jiangsu University of Technology and Science, People’s Republic of China, and Shiying Chen, Department of Material Engineering, Genex Technologies Inc., Rockville, Md. urface strengthening is an economic and powerful way to lengthen the life of a die. Many kinds of metal composite coatings have been used in the die industry. Composite coatings offer major improvements in mechanical properties such as stiffness. strength, wear resistance, and an elevated temperature operating range when compared with conventional alloys. Ni-Co-ZrO, composite coatings exhibit much better corrosion resistance, adhesion, and oxidation resistance at high temperature. They can be electroplated in a bath or by brush plating: however, brush plating is more convenient for operation in the die industry. This article discusses composite brush plating.
S
MATERIALS COMPOSITE
OF THE COATING
Nickel is the metal matrix of this composite coating. It has enough strength and hardness to hold the second-phase particles. A nickel matrix composite coating also offers oxidation and corrosion resistance at normal temperature, as does cobalt. Nickelcobalt alloy coatings exhibit much better chemical and mechanical properties than pure nickel or cobalt coatings. The alloys possess excellent resistance to wear from fretting at high temperature and are effective at extending the life of forging die molds. The electrode potential of nickel is similar to that of cobalt, so it is very easy to form Ni-Co alloy coatings with bath plating; but with brush plating, there are some technical problems, which are difficult and, to a certain degree, remain to be resolved. Experimental results show that a lot of materials can be codeposited with Ni-Co alloy as second-phase dispersions when bath plating, but there are only a few materials that can be depos46
ited with Ni-Co alloy with brush plating. ZrO, is the best. ZrO, has good chemical stability and resistance to wear at high temperature; its melting point is high (2,700”C). microhardness is 16.000 N/mm’, and the pressureresistance limit goe:s over 206 N/mm’. RESULTS
OF EXPERIMENTS
To produce a composite coating that will strengthen the surface of machine parts, both the processing parameters for brush plating and the composition of the brush-plating solution must be specified. Results of experiments show that the type and character of additions, brushplating voltage, interactive effect of voltage with concentration of main salt. relative motion speed, force between stylus and workpiece, and size of solid particles all have major effects on the quantity of ZrO, deposited. Effect of Surfactant Tests show that a cationic surfactant has a more evident effect on composite coating with brush plating than does anionic or nonionic surfactants. A small amount of cationic surfactant adhered to the surface of the ZrO, is beneficial. The content of ZrO, incorporated in the composite coating can thus be increased. Effect of Voltage Brush plating voltage has the greatest effect on the content of ZrO, incorporated. Second is the interactive effect between the voltage and the concentration of the main salt. A high concentration electrolyte will always match with high voltage. With an increase of voltage, it will increase cathode current density. so it can deposit the alloy faster than with lower voltage. Experiments when the nickel concentration is 0.75 M, 0.5 M, and 0.1 M, 0 CopyrightElsevier
Science
Inc.
respectively, in the electrolyte, result in the ZrO, particle incorporation of 6.8%. 2.0%. and 0.0%.
Effect of Particle Concentration Orthogonal tests show that zirconium dioxide content incorporation into the coating will increase along with the increase of zirconium dioxide in the electrolyte up to a value of saturation. After this point, it will decrease as shown in Figure I. It can be seenthat the saturationcontent is 6.8% by weight. which may be impossibleto exceed.
Particle Size Solid particles should not be oversized, commonly lessthan 40 pm. Experiments proved that a smooth composite coating can be obtained with particle size of I to 4 pm, In all cases participles are not only uniformly dispersedbut also set in their “bear holes” securely and wrapped in by a crystalline grain of matrix metals. Particles with too small a size will coacervate easily and deposit nonuniformly.
I 50
75
100
ZrO, in solution
(gn)
Figure 1. The interrelationship of ZrO, concentration in solution and incorporation in coating. METAL
FINISHING
l
MAY 1998
Table I. Plating Electrolyte Composition and Operating Parameters NiSO,~GH,O CoSO,.7H,O W2W7 NaCHJOO NaCl NH,.H,O PH ZrO, particles (average size, l-4 vm) Voltage Relative speed (between cathode and anode)
Wearability
270 g/L 25 @L 60 g’L 25 g/L 5-10 g/L 125 g/L 7.5 60 g/L 7.5-a v 5.5 mlmin
Test and Application
Based on the experimental results, the processand function of the composite coating were researched by means of regression analysis and a crosstest. The results demonstratethat the composite coating plated with optimum parametershas better wear resistance and a stronger binding force with its substrate. The optimum of electrolyte composition and processis listed in Table 1. By measuringlength and weight, respectively, the relative wearability of the composite coatings was compared with that for Ni-Co alloy coating, hard chromium coating, and quenched 45 steel. The matched pair specimenwas made of quenched 45 steel (HR, 5758), plain againstplain run under nonlubrication conditions. The measured results are listed in Table Il.
Table II. Comparison of Coating Wearability
Volume of water Relative wearability
Ni-Co-Zro,
Ni-Co
Chromium
Material H&51
45 Steel k/R,57
1.132 5.70
2.072 3.11
1.862 3.53
6.955 0.93
6.449 1 .oo
Measured results show that under nonlubrication conditions, the wearability of the compositecoating is 1.81 times higher than that of Ni-Co coating, 5.7 times higher than that of quenched 45 steel. and 1.61 times higher than that of lhe hard chromium coating. Microhardnessis HV 618 to 680; the largestis HV 770. The surfacerating of the specimen’soriginal surface is Ra = 1.65 ym after electroplating.The plated surface rating is Ra = 1.0 pm. and its surfacek plain, smooth,dense,no pits, no holes,anduniform in color. The binding force betweencoating and substrate i, satisfactory for forging dies, which was proved in applications in the die industry. The rolling forged rotary blade die is made of alloy steel (3Cr,W,V, HR, 54). and the material of the rotary blade is 65 Mn or 150SiMn; its forge hot temperatures are at 1.000 to I, 100°C. After composite plating on the die surface, the life of the plated die is prolonged more than one time compared with the original one, and the surfacerating and geometricsize of forged pieces are greatly improved.
Some other examplesinclude composite plating on the surfacesof PVC injection and extrusion dies. They exhibited high corrosion and wear resistance in field tests with a 3-year life.
MECHANISM OF STRENGTHENING
OF COATING
The wear resistanceof the coating hasa close relative with its texture and surface morphology. Different process parameterswill lead Ni-Co-ZrO, coatings with different morphologies.The structure of the matrix was studied using scanning electron microscopy (SEMI; photomicrographs show that they can be classified as three types, which are shown in Figure 2. They are named, respectively, (a) cauliflower shape. (b) cotton-wool shape, and (c) plate shape. The crystals of the cauliflowershapeand cotton-wool-shape composite coatings are arranged at random. and the surface is rough but exhibits outstanding wear resistanceunder the boundary lubrication condition. The plate-shapecoating has fine and dense crystals, which are arrangedregularly;
Figure 2. Scanning electron microscopy micrograph of the matrix (Ni-Co) and the diffusion of particles (ZrO,).
50
METAL FINISHING
l
MAY 1998
Figure 3. Strengthened fine crystal (400 X). the surface of coating is smooth, but the matrix has fine net cracks, as seen by SEM micrograph. This coating has higher wear resistance than the others under the nonlubrication condition.
Particle Support
Incorporation to Operating Load
Figure 3 goes a step further to show the surfaces of the three types of coatings just described. It can be seen that in each type of coating ZrO, particles can be deposited with Ni-Co alloy and distribute uniformly under optimum parameters. They readily bear the operating load, and the Ni-Co matrix acts for holding particles. The mechanical strength of Ni-Co alloy is higher than that of simple Ni or Co. The Ni-Co alloy coating always produces a large number of crystalloblastic crystalline grains during electroplating because of dislocations. Due to ZrO, particle codeposition, it will resist dislocation motion. Generally, dislocation cannot pass over secondphase particles, but they can bend around the second-phase particles under the action of applied force. forming
Figure 4. Nonuniform dispersion particles.
METAL FINISHING
l
MAY 1998
dislocation rings and more dislocation rings. Over and over again, a new dislocation ring is formed; it is named a “halo.” By SEM micrograph of the “halo.” it also can be seen that the halo consists of a large number of links between particle and matrix. The halo with these links is ver,y strong and the links are able to keep their structure from being broken even under enormous external loads (up to 1 million Pa). Owing to the high gripping force of these links inside a. halo, the composite coating has a higher load capability and wear resistance. It is also these links that prevent the coating from being damaged and scraped during machining and wear. Based on experiments, the hardness and wear resistance of the composite coating vary with the content of ZrO, particles. First, they increase in proportion to particles: after a certain degree, they are inversely proportional. It may be that with too much particle incorporation in the coating. rhe distance between both particles is less than a certain length and the Idislocation line cannot bend and will rapidly pass the second particle. No halo can be formed, so the strengthening effect is decreased. The effect of strengthening is also closely related to diffusion of the particles. The content of ZrO, in Figure 4 is more than that in Figure 3 by an order of magnitude, but the wear resistance of the former is low. Thus, the forger’s process parameters were not properly selected, and the matrix metals have a lot of pits and are porous.
They cannot hold the particles firmly, and particles are easily stacked in chinks or pits of the Ni-Co matrix. In the wear test the particles drop down, wearability is poor, and the wearing surface is left with deep scratches as shown in Figure 4.
Difference Coefficient
in Heat Expansion
Because the heat expansion coefficient of ceramic particles is less than that of metals, the internal stress resulting from high temperature is greatly reduced, and the heat fatigue strength is increased. Except for this, the strengthening mechanism also includes hydrogen solid solution strengthening caused by hydrogen deposition within the Ni-Co matrix, resulting in metal crystal lattice deformation.
CONCLUSION Under nonlubrication friction, the wear resistance of Ni-Co-ZrO, composite coatings is 1.81 times greater than that of the Ni-Co coating. 5.7 times greater than for 45 steel (HR,57). and 1.61 times greater than the hard chromium coating. It is proved from practice that the composite coating is characterized by satisfactory wear resistance, oxidatron resistance at high temperature. and fatigue resistance at high temperature. It is convenient to strengthen the surface of the die with brush plating. The tnajor strengthening mechanism is fine grain strengthening and secondphase particle strengthening. MC
51
S
MATERIALS COMPOSITE
OF THE COATING
Nickel is the metal matrix of this composite coating. It has enough strength and hardness to hold the second-phase particles. A nickel matrix composite coating also offers oxidation and corrosion resistance at normal temperature, as does cobalt. Nickelcobalt alloy coatings exhibit much better chemical and mechanical properties than pure nickel or cobalt coatings. The alloys possess excellent resistance to wear from fretting at high temperature and are effective at extending the life of forging die molds. The electrode potential of nickel is similar to that of cobalt, so it is very easy to form Ni-Co alloy coatings with bath plating; but with brush plating, there are some technical problems, which are difficult and, to a certain degree, remain to be resolved. Experimental results show that a lot of materials can be codeposited with Ni-Co alloy as second-phase dispersions when bath plating, but there are only a few materials that can be depos46
ited with Ni-Co alloy with brush plating. ZrO, is the best. ZrO, has good chemical stability and resistance to wear at high temperature; its melting point is high (2,700”C). microhardness is 16.000 N/mm’, and the pressureresistance limit goe:s over 206 N/mm’. RESULTS
OF EXPERIMENTS
To produce a composite coating that will strengthen the surface of machine parts, both the processing parameters for brush plating and the composition of the brush-plating solution must be specified. Results of experiments show that the type and character of additions, brushplating voltage, interactive effect of voltage with concentration of main salt. relative motion speed, force between stylus and workpiece, and size of solid particles all have major effects on the quantity of ZrO, deposited. Effect of Surfactant Tests show that a cationic surfactant has a more evident effect on composite coating with brush plating than does anionic or nonionic surfactants. A small amount of cationic surfactant adhered to the surface of the ZrO, is beneficial. The content of ZrO, incorporated in the composite coating can thus be increased. Effect of Voltage Brush plating voltage has the greatest effect on the content of ZrO, incorporated. Second is the interactive effect between the voltage and the concentration of the main salt. A high concentration electrolyte will always match with high voltage. With an increase of voltage, it will increase cathode current density. so it can deposit the alloy faster than with lower voltage. Experiments when the nickel concentration is 0.75 M, 0.5 M, and 0.1 M, 0 CopyrightElsevier
Science
Inc.
respectively, in the electrolyte, result in the ZrO, particle incorporation of 6.8%. 2.0%. and 0.0%.
Effect of Particle Concentration Orthogonal tests show that zirconium dioxide content incorporation into the coating will increase along with the increase of zirconium dioxide in the electrolyte up to a value of saturation. After this point, it will decrease as shown in Figure I. It can be seenthat the saturationcontent is 6.8% by weight. which may be impossibleto exceed.
Particle Size Solid particles should not be oversized, commonly lessthan 40 pm. Experiments proved that a smooth composite coating can be obtained with particle size of I to 4 pm, In all cases participles are not only uniformly dispersedbut also set in their “bear holes” securely and wrapped in by a crystalline grain of matrix metals. Particles with too small a size will coacervate easily and deposit nonuniformly.
I 50
75
100
ZrO, in solution
(gn)
Figure 1. The interrelationship of ZrO, concentration in solution and incorporation in coating. METAL
FINISHING
l
MAY 1998
Table I. Plating Electrolyte Composition and Operating Parameters NiSO,~GH,O CoSO,.7H,O W2W7 NaCHJOO NaCl NH,.H,O PH ZrO, particles (average size, l-4 vm) Voltage Relative speed (between cathode and anode)
Wearability
270 g/L 25 @L 60 g’L 25 g/L 5-10 g/L 125 g/L 7.5 60 g/L 7.5-a v 5.5 mlmin
Test and Application
Based on the experimental results, the processand function of the composite coating were researched by means of regression analysis and a crosstest. The results demonstratethat the composite coating plated with optimum parametershas better wear resistance and a stronger binding force with its substrate. The optimum of electrolyte composition and processis listed in Table 1. By measuringlength and weight, respectively, the relative wearability of the composite coatings was compared with that for Ni-Co alloy coating, hard chromium coating, and quenched 45 steel. The matched pair specimenwas made of quenched 45 steel (HR, 5758), plain againstplain run under nonlubrication conditions. The measured results are listed in Table Il.
Table II. Comparison of Coating Wearability
Volume of water Relative wearability
Ni-Co-Zro,
Ni-Co
Chromium
Material H&51
45 Steel k/R,57
1.132 5.70
2.072 3.11
1.862 3.53
6.955 0.93
6.449 1 .oo
Measured results show that under nonlubrication conditions, the wearability of the compositecoating is 1.81 times higher than that of Ni-Co coating, 5.7 times higher than that of quenched 45 steel. and 1.61 times higher than that of lhe hard chromium coating. Microhardnessis HV 618 to 680; the largestis HV 770. The surfacerating of the specimen’soriginal surface is Ra = 1.65 ym after electroplating.The plated surface rating is Ra = 1.0 pm. and its surfacek plain, smooth,dense,no pits, no holes,anduniform in color. The binding force betweencoating and substrate i, satisfactory for forging dies, which was proved in applications in the die industry. The rolling forged rotary blade die is made of alloy steel (3Cr,W,V, HR, 54). and the material of the rotary blade is 65 Mn or 150SiMn; its forge hot temperatures are at 1.000 to I, 100°C. After composite plating on the die surface, the life of the plated die is prolonged more than one time compared with the original one, and the surfacerating and geometricsize of forged pieces are greatly improved.
Some other examplesinclude composite plating on the surfacesof PVC injection and extrusion dies. They exhibited high corrosion and wear resistance in field tests with a 3-year life.
MECHANISM OF STRENGTHENING
OF COATING
The wear resistanceof the coating hasa close relative with its texture and surface morphology. Different process parameterswill lead Ni-Co-ZrO, coatings with different morphologies.The structure of the matrix was studied using scanning electron microscopy (SEMI; photomicrographs show that they can be classified as three types, which are shown in Figure 2. They are named, respectively, (a) cauliflower shape. (b) cotton-wool shape, and (c) plate shape. The crystals of the cauliflowershapeand cotton-wool-shape composite coatings are arranged at random. and the surface is rough but exhibits outstanding wear resistanceunder the boundary lubrication condition. The plate-shapecoating has fine and dense crystals, which are arrangedregularly;
Figure 2. Scanning electron microscopy micrograph of the matrix (Ni-Co) and the diffusion of particles (ZrO,).
50
METAL FINISHING
l
MAY 1998
Figure 3. Strengthened fine crystal (400 X). the surface of coating is smooth, but the matrix has fine net cracks, as seen by SEM micrograph. This coating has higher wear resistance than the others under the nonlubrication condition.
Particle Support
Incorporation to Operating Load
Figure 3 goes a step further to show the surfaces of the three types of coatings just described. It can be seen that in each type of coating ZrO, particles can be deposited with Ni-Co alloy and distribute uniformly under optimum parameters. They readily bear the operating load, and the Ni-Co matrix acts for holding particles. The mechanical strength of Ni-Co alloy is higher than that of simple Ni or Co. The Ni-Co alloy coating always produces a large number of crystalloblastic crystalline grains during electroplating because of dislocations. Due to ZrO, particle codeposition, it will resist dislocation motion. Generally, dislocation cannot pass over secondphase particles, but they can bend around the second-phase particles under the action of applied force. forming
Figure 4. Nonuniform dispersion particles.
METAL FINISHING
l
MAY 1998
dislocation rings and more dislocation rings. Over and over again, a new dislocation ring is formed; it is named a “halo.” By SEM micrograph of the “halo.” it also can be seen that the halo consists of a large number of links between particle and matrix. The halo with these links is ver,y strong and the links are able to keep their structure from being broken even under enormous external loads (up to 1 million Pa). Owing to the high gripping force of these links inside a. halo, the composite coating has a higher load capability and wear resistance. It is also these links that prevent the coating from being damaged and scraped during machining and wear. Based on experiments, the hardness and wear resistance of the composite coating vary with the content of ZrO, particles. First, they increase in proportion to particles: after a certain degree, they are inversely proportional. It may be that with too much particle incorporation in the coating. rhe distance between both particles is less than a certain length and the Idislocation line cannot bend and will rapidly pass the second particle. No halo can be formed, so the strengthening effect is decreased. The effect of strengthening is also closely related to diffusion of the particles. The content of ZrO, in Figure 4 is more than that in Figure 3 by an order of magnitude, but the wear resistance of the former is low. Thus, the forger’s process parameters were not properly selected, and the matrix metals have a lot of pits and are porous.
They cannot hold the particles firmly, and particles are easily stacked in chinks or pits of the Ni-Co matrix. In the wear test the particles drop down, wearability is poor, and the wearing surface is left with deep scratches as shown in Figure 4.
Difference Coefficient
in Heat Expansion
Because the heat expansion coefficient of ceramic particles is less than that of metals, the internal stress resulting from high temperature is greatly reduced, and the heat fatigue strength is increased. Except for this, the strengthening mechanism also includes hydrogen solid solution strengthening caused by hydrogen deposition within the Ni-Co matrix, resulting in metal crystal lattice deformation.
CONCLUSION Under nonlubrication friction, the wear resistance of Ni-Co-ZrO, composite coatings is 1.81 times greater than that of the Ni-Co coating. 5.7 times greater than for 45 steel (HR,57). and 1.61 times greater than the hard chromium coating. It is proved from practice that the composite coating is characterized by satisfactory wear resistance, oxidatron resistance at high temperature. and fatigue resistance at high temperature. It is convenient to strengthen the surface of the die with brush plating. The tnajor strengthening mechanism is fine grain strengthening and secondphase particle strengthening. MC
51