Tribological properties evaluation of NiP coating manufactured with electroless plating on aluminum alloy substrate

Results of tribological tests of Ni-P coating were presented in following paper. Tests were conducted with tribotester device using linear reciprocating wear test. Tribological wear test conditions, especially speed, load and sample geometry were described. Characteristics of coefficient of friction variation during wear testes were recorded. Conclusions included results interpretation especially focusing on influence of coating thickness in its properties.


Introduction
Aluminum alloys are nowadays widely used among all branches of industry, including automotive (engines, performance cars), marine (hulls), railroad vehicles (carriage structure), and aviation (frames, equipment).This increasing demand is connected with properties of these materials-especially low weight and corrosion resistance, while compared with steel.Among all aluminum alloys, EN AW 7075 (AlZn5.5MgCu)presents very satisfying mechanical properties.Still, lot of research is conducted on field of its weldability and other joining methods [1].However, steel can be recognized as more common material, mainly thanks to its strength, lower material cost and resistance to abrasive wear.What is more, mechanical properties and wear resistance of both carbon and alloy steels can be easily increased by heat treatment or application of anti-wear coatings.Still, properties of several aluminum alloys can be modified by both heat treatment and coating application, in limited range.[2].Unfortunately, it's impossible to adopt same coatings and methods that are commonly used for steel, to aluminum alloys.This is caused by significant difference in material properties, especially hardness and surface adhesion.Also, application of same technologies that are applicable for steel, is not valid for aluminum alloys because of lower processing temperatures.
However, among known engineering solutions, several coatings can be successfully used for steel and aluminum alloy substrates.Ni-P (nickel-phosphorus) coating manufactured with electroless, chemical reduction method can be applied to aluminum alloy substrate [3].In contrast to electric manufactured coatings, electroless Ni-P coating can be applied not only for decorative reasons, because of better adhesion to substrate and higher resistance to abrasive wear [4].
Moreover, mechanical properties of these coatings can be improved by composition modification, e.g. with ceramic additives, what makes them composite coatings.Nano dispersive additives of PCD (poly-crystalline diamond), corundum, silicon carbide, silicon nitride, PTFE (Polytetrafluoroethylene) can be used, in order to prepare Ni-P coating for actual application.Both hardness increase and wear resistance can be achieved by proper composition modification with additives, what can be very promising, e.g. for applications were dry friction can occur [4][5].
Ni-P coating described in following paper should be considered as basic coating without any additives or modifications.Main aim of research presented above is to demine tribological properties of Ni-P coating manufactured on AW-0705 aluminum alloy.
Conducted tribological test were also aimed to determine and evaluate possibilities of Ni-P electroless coating application on aluminum alloy substrate in specific technical applications.What is more, very interesting results were achieved by combining such coatings with application of innovative lubricants with additives.Numerous records among state of art proved usefulness of Ni-P coating on steel substrates, as a successful method for increasing wear resistance [5][6].This allows to make an assumption, that further research on field of application of electroless Ni-P coating on different substrates should be considered.

Overview of electroless Ni-P coating properties
Nickel-phosphorus coatings are amorphous, homogenous, hard, fragile, self-lubricating, vulnerable to soldering and corrosion resistant.Because of that, they can be applied on wide variety of fields.Ni-P coatings manufactured with electroless methods because of high hardness show major wear resistant directly after plating and also after heat treatment.Also because of its amorphous nature, Ni-P coating can be considered as good heat barrier [7].
While compared with nickel based coatings manufactured with galvanic techniques, electroless plated Ni-P coating presents following advantages [8][9]: -Nickel coating tend to create coating of homogenous thickness, on numerous surfaces exposed to coating solution, regardless to shape and position of coated piece, -Coating structure is homogenous and independent from substrate surface structure, -Higher hardness that can be increased by further heat treatment, -Higher corrosion resistance, -Can be manufactured directly on aluminum substrate, -Can be easily soldered.
As electroless plating widens spectrum of possible application of nickel based coatings, also presents some disadvantages, mainly connected with economic aspect [8.9]: -Chemical compound used during electroless plating are far more expensive that these used in galvanic processes, -Regeneration and preparation of coating solution is complicated and expensive process.
Basing on coating properties, presented advantages and disadvantages, and previous research, it can be stated that described Ni-P coating can be very promising solution for increasing wear resistance of parts made of AW-7075 aluminum, and should be subject of tribological tests.

Testing methodology
Samples consisting of AW-7075 aluminum substrate with Ni-P coating manufactured with electroless chemical method was tested.Chemical composition of substrate alloy is presented in Table 1.20 mm x 20 mm x7 mm samples were cut of a 120 mm AW-7075 aluminum disc.Samples were machined to achieve perpendicular faces and desired roughness.Ni-P coating of various thickness was plated on samples.Good adhesion between coating and substrate was achieved thanks to surface etching, which helped in oxides removal.After surface preparation, Ni-P coating were produced.During described test, coating of 20 μm and 10 μm in thickness were applied.
Tribological test were conducted with UMT-2 Brucker device.Tool with bearing ball was used for testing.Ball was made of chromium stainless steel of 62 HRC hardness, according to PN-EN ISO 4527:2005.Diameter of balls used in testing was 6.32 mm (1/4 inch).Bearing ball was placed in holder, and fixed to avoid rotation.Ball was pushed towards tested sample.Reciprocating movement of tested sample was introduced.Sample was experiencing linear reciprocating movement of specific speed and distance.Thanks to this configuration of tested sample and tool (bearing ball), contact zone was circle (according to Hertzian theorem).Because of applied kinematics, wear trace was expected to be a single line.Whole test could be described as common pin-on-flat tribological test.
Sample with coating prepared earlier were loaded normally to its surface via bearing ball with force of 5 N. Simultaneously, reciprocating linear movement was introduced.Because of lubricant absence, test conditions are resized as dry friction.Single testing cycle consisted of two strokes on distance of 10 mm.Testing cycle started and stopped in same point.Speed of sample during test was set to 10 mm/s.Duration and number of cycles was different for each sample.Test was terminated after recognition of coating breakage.
Thanks to device design, applied force of 5 N stayed constant during test.Device was equipped with force sensor and regulator.If force measurement and regulation would be omitted, load force would decrease during test because of material wear.In described tests, force was regulated with precision of +/-0.5 N.Such low value of force was assumed, as substrate was made of soft material.During test performed with hard substrates, greater forces can be applied.Device used in test is capable of applying axial forces of 200 N.
Before each test, tested samples were cleaned.Data acquainted during test covered: time [s], frictional force FT [N], acoustic emission level AE [V] and also coefficient of friction-μ.Device was equipped with two dimensional force sensor, which allowed to measure force in direction of load and in direction of sample movement.Basing in this values, coefficient of friction could be calculated.
For each test, frictional force variation was recorded.Because of the reciprocating movement, frictional force was alternating.To allow further processing of results and values, signal for frictional force was periodically filtered in order to achieve positive values.As well, peaks occurring during movement direction change were filtered.In total, two samples were tested.Thickness of coating for sample I was 0.02 mm and for sample II-0.01 mm.

Results presentation and discussion
Samples were evaluated regarding to recorded coefficient of friction and variations of acoustic emission during test.Data acquisition and processing were performed with UMT Brucker laboratory software.Frequency of data acquisition was 100 Hz.Precision of force measurement was 0N.Precision of acoustic emission measurement was 0.05 V. Figure 1 and 3 present variations of coefficient of friction during tribological tests.AE variation are shown in Figure 2 and 4, respectively.Interpretation of these results was described in following chapter.

Conclusions
Obtained results allowed to evaluate resistance to abrasive wear of Ni-P coated samples.Two samples were tested, with various coating thickness.All results presented by graphs in Figure 1-4 share similar characteristics.In case of coefficient of friction, monotonous rise of recorded magnitude is recorded in first phase of experiment.After certain number of cycles, rapid increase in COF magnitude occurred.After this peak, magnitude of COF starts to fluctuate around specific value.In case of coating of lower thickness, fluctuations present greater amplitude, when compared with results for thicker coating.Minor peaks in the beginning of test can be described as noise caused by initiative contact between testing tool and sample.AE variations can be also considered as consisting of two phases.In first phase acoustic emission is constant and low.Minor monotonic increase can be observed shortly before end of first phase.Second phase, analogically as in case of coefficient of friction, begins with rapid increase in AE magnitude.After this peak, acoustic emission fluctuates at high levels.As well, minor peak of AE level is observed in beginning of test.Analogically to COF measurement, this peak is considered as noise caused by initial contact.
As point of rapid rise is coherent for both acoustic emission and coefficient of friction and in case of both samples, it can be stated that in that point breakage of coating occurs.Low values of AE during first phase of experiment may be connected with abrasive wear of coating.Rapid increase of both AE and COF is caused by breakage of coating and beginning of friction between substrate and testing tool.Great fluctuations and significant value of AE is caused by relatively high roughness of substrate surface, because of itching applied before coating plating.
Conducted tests allowed to evaluate influence of coating thickness on durability of Ni-P coating in pin-on-flat tribological test.Obtained results show that coating of thickness equal to 20 μm was able to carry load and friction for about 400 seconds before observable coating breakage.This value is two times higher, when compared with results obtained for 10 μm coating, which broke after 200 seconds from test beginning.
Value of kinetic coefficient of friction for first phase of experiment was varying from 0.1 to 0.3, and after coating breakage reached 0.6 which proves that applied coating helps in decreasing value of coefficient of friction.Stable and low AE signal in first phase of coating wear proves that wear processes that occurs during coating wear are stabilized and no intense destructive phenomena are present, like e.g.adhesive wear.This means that wear of Ni-P coating in described test is a predictable process, and final, destructive phase of wear can be anticipated.In case of machine maintenance this is considered as significant advantage, as lesser probability of unexpected and unpredictable failures means easier machine maintenance.