Process and bath for the production of an article plated with boron carbide in a nickel-phosphorus matrix

Abstract: Described herein is an article plated with boron carbide in a nickel-phosphorus matrix, obtained by means of a plating process comprising the following steps: a) preparation of an electrolytic bath comprising two or more nickel salts, at least one complexing agent, at least one phosphorus salt, and, in addition, an anti-tensioning agent and boron carbide in the form of powder; b) electroplating of the article in said electrolytic bath at a temperature ranging from 40°C to 70°C, with a current density ranging from 1 to 10 A/dm 2 and finally c) heat treatment of the product thus plated. The aforesaid plating is used for any article that requires a type of plating which presents high resistance to wear, and in particular for cylinders for the production of corrugated cardboard.

Description: The present invention refers to an electrolytic bath for depositing a plating of boron carbide in a nickel-phosphorus matrix, and to the process of preparation of an article by electrodeposition of inert particles in a nickel-phosphorus matrix. In the sector of the production of articles that are subjected to severe wear and, more in particular, in the sector for producing cylinders for the production of corrugated cardboard, the role of the plating applied on the article, and in particular of the plating applied on the cylinder, is of fundamental importance. In fact, it is precisely the plating that must be able to withstand severe wear. There are two types of plating currently used in the said sector: classic chrome plating, with thicknesses of the order of 100 µm, deposited electrolytically, and tungsten-carbide plating. Albeit having an acceptable cost, classic chrome plating presents a number of drawbacks. In the first place, it is not possible to obtain uniformity of plating; namely, plating is deficient in the groove and abundant on the sharp point on account of the point effect. As a result, the thickness in the groove may be 70% less than the thickness on the sharp point, even though an appreciable hardness is maintained (approximately 850 Hv). The second type of plating used in the sector of plating of articles that undergo severe wear, such as cylinders for the production of corrugated cardboard, is tungsten-carbide plating. Albeit having good characteristics of hardness (approximately 1300 Hv), good uniformity, and good adhesion, this type of plating presents very high costs which render this type of solution far from economically convenient and not usable for a large number of industrial applications. JP 62 161995 discloses a hard steel wire or alloy steel wire on the surface of which hard composite plated layer having the matrix of Ni-alloy containing co-deposit hard fine particles, Ni and P are formed. US-A-4,666,786 discloses a composite nickel plated sliding surface obtained by the formation of a composite nickel plating film on a sliding surface of an automobile part by electroless nickel plating. Now according to the present invention, an electrolytic bath and a process have been found which enable the drawbacks of the prior art to be overcome. In particular, a subject of the present invention is an electrolytic bath as defined in claim 1. Yet another subject of the present invention is a process for the production of a plating with boron carbide in a nickel-phosphorus matrix as defined in claim 8. In particular, the electrolytic bath is used for the plating treatment of articles to obtain plated articles having the desired quality. The main advantage of the plating process according to the present invention consists in that easy management of the entire process is combined with a low cost, and hence excellent possibility of application in the industrial field, and at the same time a plated product is obtained that is provided with high resistance to wear. In addition, a further advantage of the plating obtained using the process according to the present invention lies in its excellent adhesion to the surface of the article to be plated. The electrolytic bath according to the present invention may comprise, as nickel salts, nickel sulphate and nickel chloride. The electrolytic bath contains, further as phosphorus salt, salts of phosphorous acid. The complexing agents are chosen from among citric acid, lactic acid, malic acid, malonic acid, succinic acid, glycolic acid, and short-chain carboxylic acids. Preferably, the complexing agent is lactic acid. The anti-tensioning agents are chosen between saccharin and compounds of diethylcarbamic acid, preferably saccharin. Preferably, the electrolytic bath according to the present invention contains from 0.01 mol. to 3 mol. of nickel sulphate, from 0.003 mol. to 2 mol. of nickel chloride, from 0.006 mol. to 1.8 mol. of the salt of phosphorous acid, from 0.05 mol. to 2 mol. of lactic acid, from 0.2 g/l to 30 g/l of saccharin, and from 0.2 g/l to 30 g/l of boron carbide. More preferably still, the electrolytic bath according to the present invention contains from 0.1 mol. to 2 mol. of nickel sulphate, from 0.01 mol. to 1.5 mol. of nickel chloride, from 0.06 mol. to 1.0 mol. of the salt of phosphorous acid, from 0.1 mol. to 1 mol. of lactic acid, from 3 g/l to 10 g/l of saccharin, and from 5 g/l to 15 g/l of boron carbide. In particular, the particles of boron carbide have a grain size of between 3 and 6 µm. The said bath can be used with a current density of from 1 to 10 A/dm 2 , at a temperature ranging between 40°C and 70°C, under stirring, whilst the pH value of the electrolytic bath may range from 0.4 to 10. The temperature of the bath and the current density for the plating process have been chosen in the range referred to above for the reasons explained in what follows. At temperatures lower than 40°C, the current density would not be sufficient, and there would be a low efficiency of electrodeposition. At temperatures higher than 70°C the disadvantage of the high evaporation of the bath would exceed the advantage due to an increase in the efficiency of electrodeposition. Step b) of the plating process according to the present invention is preferably performed at a temperature of 60°C, with a current density of 2 A/dm 2 , under stirring. Step c) is performed at a temperature within the 250°C - 400°C range, and preferably at a temperature of 340°C, for 12 hours. The material for the cathode is the material to be plated, whilst the anode can be chosen from among anodes made of electrolytic nickel. In particular, a plated article obtained with the process according to the present invention is a cylinder for the production of corrugated cardboard. The aforesaid plating can, in any way, be used for any type of article which, in any sector of application, requires plating that is resistant to wear, such as aluminium articles. The plated article obtained according to the present invention may undergo further treatments, such as a polishing treatment using diamond paste. In the particular case of an article having a poorly receptive surface, such as a nitrided surface, in order to obtain better adhesion of the plating with boron carbide in a nickel-phosphorus matrix, the plating process according to the invention may be advantageously preceded by the following pre-treatment steps: 1) sanding; 2) chemical degreasing; 3) washing; 4) neutralization in 10% sulphuric acid; 5) washing; 6) deposition of chemical nickel from an alkaline solution at 35-43°C, the said solution being known under the commercial name ENPLATE AL 100; and 7) further washing. In particular, sanding may be performed using a machine at a pressure of 7 bar and employing corundum with grain size 150; chemical degreasing may be carried out using ultrasound (6 W per litre) at a temperature of 75°C; the first washing may be carried out in purified water circulated on activated carbon, whilst the washing referred to in point 4) is carried out in purified water circulated on dolomite; deposition of chemical nickel is carried out in cold conditions, keeping the pH value above 9.6 to prevent formation of clouding of the solution and of dark deposits; the final washing consists of a first static washing in deionized water and of a second washing in demineralized water circulated on resins. The plating process according to the invention is then applied on the product thus obtained. The characteristics and advantages of the product according to the present invention will emerge more clearly from the ensuing detailed description, which is given purely to provide non-limiting examples. Example 1 A plating film consisting of particles of boron carbide in a nickel-phosphorus matrix having a mean thickness of 120 µm was deposited starting from an electrolytic bath containing 72.6 g/l of nickel sulphate, 6 g/l of nickel chloride, 10 g/l of potassium phosphite, 45 g/l of 90% lactic acid, 5.8 g/l of saccharin, and 10 g/l of boron carbide 1500, at a temperature of 60°C, with a maximum current density of 4 A/dm 2 , under mechanical stirring for 180 minutes. The hardness of the plating obtained was found to be 650 Hv. After treatment at 340°C for 12 hours, the plating was found to have a hardness of 950 Hv. The total phosphorus present in the plating was 2.5% determined under a scanning electron microscope, and the boron carbide present in the plating was 35 vol%. There was a contained level of evaporation, and tensioning was altogether absent. Example 2 A plating film consisting of particles of boron carbide in a nickel-phosphorus matrix having a mean thickness of 120 µm was deposited starting from an electrolytic bath containing 72.6 g/l of nickel sulphate, 10 g/l of nickel chloride, 10 g/l of potassium phosphite, 80 g/l of 90% lactic acid, 5.8 g/l of saccharin, and 10 g/l of boron carbide 1500, at a temperature of 60°C, with a maximum current density of 4 A/dm 2 , under mechanical stirring for 180 minutes. The characteristics of the plating obtained were equivalent to those described for the product of Example 1. Example 3 A plating film consisting of particles of boron carbide in a nickel-phosphorus matrix having a mean thickness of 120 µm was deposited starting from an electrolytic bath containing 72.6 g/l of nickel sulphate, 3 g/l of nickel chloride, 5 g/l of potassium phosphite, 90 g/l of 90% glycolic acid, 5.8 g/l of saccharin, and 10 g/l of boron carbide 1500, at a temperature of 60°C, with a maximum current density of 4 A/dm 2 , under mechanical stirring for 180 minutes. The characteristics of the plating obtained were equivalent to those described for the product of Example 1. Example 4 (comparative ) A plating film consisting of particles of boron carbide in a nickel-phosphorus matrix having a mean thickness of 120 µm was deposited starting from an electrolytic bath containing 66 g/l of nickel sulphate, 12 g/l of nickel chloride, 5 g/l of phosphorous acid, 45 g/l of 90% lactic acid, and 6 g/l of boron carbide, at a temperature of 70°C, at a pH of 4, and at a current density of 2 A/dm 2 , for 120 minutes. In these conditions, excessive anodic etching was found which was due to the excessively high content of chlorides, a low presence of inert particles - in fact, the boron carbide present was 23.4 vol% -, excessive evaporation due to the excessively high temperature, severe internal tensioning, and low deposition rate. Also this electroplated specimen underwent heat treatment at 340°C for 12 hours. The hardness of the plating obtained was found to be 1050 Hv. The total thickness of the plating was 66 µm, with a thickness in the groove of 44 µm. There was thus a difference in thickness between the sharp point and the groove of 33.4%. The total phosphorus present in the plating was 4.7% determined under a scanning electron microscope. Tensioning, due to the absence of saccharin, was very evident in the specimen. Example 5 A wear test was carried out comparing the plating obtained according to Example 1 with a plating consisting of electrolytic chromium and with a tungsten-carbide plating deposited using the "super detonation gun" technique. The test was carried out by putting the three test specimens simultaneously on a metallographic lapping machine and using, as abrasive means, a 1200-type lapping paper with water at room temperature, loading for 4 minutes with a force of 4030 kg. The comparison was made by measuring the thickness before and after the test by means of an optical micrometer at x800. The following results were obtained: The plating of Example 1 in nickel-phosphorus plus boron carbide presented wear for a thickness of 45 µm; The tungsten-carbide plating presented wear for a thickness of 30 µm; The electrolytic-chromium plating presented wear for a thickness of 65 µm. It was therefore found that, setting the wear undergone by the tungsten plating equal to zero, corresponding to a wear resistance equal to one hundred, the plating in nickel and phosphorus with boron carbide underwent twice the wear of the tungsten plating and presented a wear resistance 66% higher than that of an electrolytic-chromium plating. It was therefore surprisingly found that boron carbide, by contributing to lowering the friction coefficient, makes it possible to achieve a higher wear resistance of the plating as compared to an electrolytic-chromium plating. It was moreover found that, after approximately 12 hours of operation, the chemical parameters of the electrolytic solution or bath no longer fell within the pre-defined values. This applied in particular to the pH and to the metallic nickel. After 12 hours it is therefore necessary to treat the electrolytic solution, allowing the particles to settle for 24 hours and restoring the level by treating with a solution containing ammonium sulphate, ammonium chloride, phosphorous acid, lactic acid, potassium hydrate, and saccharin. The main advantage of the plating process according to the present invention hence lies in combining easy management of the entire process with a low, cost, and hence with an excellent possibility of industrial application, at the same time obtaining a plated article provided with a high level of wear resistance. Furthermore, another advantage of the plating obtained with the process according to the present invention consists in the excellent adhesion that it presents to the highly critical surface of the article to be plated.

Claims: An electrolytic bath for depositing a plating of boron carbide in a nickel-phosphorus matrix, which comprises two or more nickel salts, at least one complexing agent chosen from among citric acid, lactic acid, malic acid, malonic acid, succinic acid, glycolic acid, and short-chain carboxylic acids, and at least one salt of phosphorus acid, characterized in that it contains an anti-tensioning agent chosen between saccharin and compounds of diethylcarbamic acid, and boron carbide in the form of powder. An electrolytic bath according to Claim 1, characterized in that the nickel salts are nickel sulphate and nickel chloride. An electrolytic bath according to Claim 4 1, characterized in that the complexing agent is lactic acid. An electrolytic bath according to Claim 1, characterized in that the anti-tensioning agent is saccharin. An electrolytic bath according to Claim 1, characterized in that it contains from 0.01 mol. to 3 mol. of nickel sulphate, from 0.003 mol. to 2 mol. of nickel chloride, from 0.006 mol. to 1.8 mol. of a salt of phosphorous acid, from 0.05 mol. to 2 mol. of lactic acid, from 0.2 g/l to 30 g/l of saccharin, and from 0.2 g/l to 30 g/l of boron carbide. An electrolytic bath according to Claim 5, characterized in that it contains from 0. 1 mol. to 2 mol. of nickel sulphate, from 0.01 mol. to 1.5 mol. of nickel chloride, from 0.06 mol. to 1.0 mol. of the salt of phosphorous acid, from 0.1 mol. to 1 mol. of lactic acid, from 3 g/l to 10 g/l of saccharin, and from 5 g/l to 15 g/l of boron carbide. An electrolytic bath according to Claim 1, characterized in that the particles of boron carbide have a grain size of between 3 and 7 µm. A process for the production of a plating of boron carbide in a nickel-phosphorus matrix, characterized in that it comprises the following steps: a) preparation of an electrolytic bath comprising two or more nickel salts, at least one complexing agent, at least one phosphorus salt, and, in addition, an anti-tensioning agent and boron carbide in the form of powder; b) electroplating of an article in said electrolytic bath at a temperature ranging from 40°C to 70°C, with a current density ranging from 1 to 10 A/dm 2 under stirring, and c) heat treatment of the product thus plated. A process according to Claim 8, characterized in that Step b) is performed at a temperature of 60°C, with a current density of 2 A/dm 2 , under stirring. A process according to Claim 8, characterized in that Step c) is performed at a temperature of between 250°C and 400°C. A process according to Claim 8, characterized in that Step c) is performed at a temperature of 340°C, for 12 hours. A process according to Claim 8, characterized in that , prior to Step a), it envisages the following pre-treatment steps: 1) sanding; 2) chemical degreasing; 3) washing; 4) neutralization in 10% sulphuric acid; 5) washing; 6) deposition of chemical nickel from an alkaline solution at 35-43°C, the said solution being known under the commercial name ENPLATE AL 100; and 7) further washing. A process according to Claim 8, characterized in that the deposition of chemical nickel is carried out in cold conditions, keeping the pH value above 9.6. A process according to claim 8, characterized in that the electrolytic bath contains, as phosphorus salt, a salt of phosphorous acid or a salt of hypophosphorous acid. A process according to claim 8, characterized in that in the electrolytic bath the complexing agent is chosen from among citric acid, lactic acid, malic acid, malonic acid, succinic acid, glycolic acid, and short-chain carboxylic acids. A process according to claim 8, characterized in that in the electrolytic bath the anti-tensioning agent is chosen between saccharin and compounds of diethylcarbamic acid.

Inventors: BURATTI ANGELO, BURATTI, ANGELO
Applicants: SBR S R L, SBR S.R.L.
Application year: 2000
Main classification: C25D15
IPC codes: C25D15, C25D5, B31F1, C25D7, C25D3