The Materials Science of Coatings and Substrates Part 12 pot

A general rule of thumb is that tensile stresses in the deposits are deleterious, and the higher the stress the worse the situation in regards to fatigue strength of the substrate.. Typ

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INTRODUCTION

Stresses which remain in components following production may be

so high that the total of operating and residual stress exceeds the material's strength Among the most famous examples of failures due to residual stresses were the all-welded Liberty ships of World War 11 (1) Over 230 of these ships were condemned because of fractures arising from failures below the design strength; these were directly due to the existence of unrelieved residual stresses set up during the welding One T-2 tanker, the

"Schenectady", had the unenviable distinction of breaking in half while being fitted out at the pier in calm seas during mild weather and without ever having "gone to sea" Investigation showed that the maximum bending moments from the loading at the time it broke up were under one-half those allowed for in the design; it had failed because it was severely over-stressed

by residual stresses alone (1) Although examples from coatings are not as dramatic, residual stresses introduced as a result of the deposition process can create problems

A residual stress may be defined as a stress within a material which

is not subjected to load or temperature gradients yet remains in internal equilibrium Residual stresses in coatings can cause adverse effects on properties They may be responsible for peeling, tearing, and blistering of the deposits; they may result in warping or cracking of deposits; they may reduce adhesion, particularly when parts are formed after plating and may alter properties of plated sheet Stressed deposits can be considerably more reactive than the same deposit in an unstressed state This point is clearly shown in Figure 1 which compares the reaction of highly stressed and

279

Figure 1: Reaction of rhodium deposits with different stresses upon exposure to nitric acid solution Deposit on the left had a tensile stress of

690 MPa while that on the right had a compressive stress of 17 MPa From reference 2 Reprinted with permission of ASM International

slightly compressively stressed rhodium deposits to nitric acid Silver coupons were plated with 5 pm (0.2 mil) thick rhodium deposits In one case the stress in the rhodium was 690 MPa (l00,OOO psi) tensile while the other was 17 MPa (2500 psi) compressive Once released from the restraining substrate by dissolution of the silver in nitric acid, the highly stressed rhodium exhibited catastrophic failure (2)

Occasionally stress may serve a useful purpose For example, in the production of magnetic films for use in high speed computers, stress in electrodeposited iron, nickel, and cobalt electrodeposits will bring about preferred directions of easy magnetization and other related effects (2)

THERMAL, RESIDUAL, AND STRESS DURING SERVICE

Two kinds of stress exist in coatings: differential thermal stress and, residual or intrinsic stress (3) Differential thermal stresses can be

calculated For example, assuming a twofold difference in coefficient of expansion between the basis metal and the coating (the differences are usually smaller), a temperature change of 100°C will produce stresses on the order of 69 MPa (l0,OOO psi) to 207 MPa (30,000 psi) (4) An electroless nickel deposit will shrink about 0.1 percent when cooled from a plating solution temperature of 90°C to ambient temperature (5)(6) Depending on the thermal coefficient of expansion of the substrate, the stress induced in the coating can be either tensile or compressive Heat treating electroless nickel deposits above 250°C increases the tensile stress due to the volume shrinkage that occurs during nickel phosphide precipitation and nickel crystallization (5) More information on this is included in the chapter on Structure

Besides differential thermal stress and stress from the coating process, an added stress can be introduced during use of the plated part An

example is gold plated spectacle frames One source of corrosive attack on plated surfaces is the formation of cracks, thereby exposing the substrate Corrosion of spectacle frames can occur due to attack of perspiration through cracks which develop if the sum of the tensile stresses in the metal exceeds the tensile strength of the plating In addition to thermal and residual stresses, a stress component can result from service usage Bending

or twisting of the plated spectacle frames can cause tensile stresses in the convex layers (7) Table 1 summarizes the stresses that might occur with these parts When the combined tensile stresses exceed the tensile strength

of the plating, cracks can develop and these expose the basis metal to corrosive attack The data in Table 1 indicate that in this situation there seems to be no danger of cracking even if all three effects take place simultaneously since the tensile strength of the gold is not exceeded (7)

Table 1 Stress Data for a Gold-Nickel Deposit on a Spectacle Framed)"

Inner stress of plating

Table 2 provides data on the relative magnitude of stresses in electrodeposits It's interesting to note that there is an apparent relationship between stress and melting point with the transition metals exhibiting the highest tensile stresses (2) Tensile stress (+) causes a plated strip to bend

in the direction of the anade; this type of bending is met when the deposit

is distended and tends to reduce its volume A plated strip that bends away from the anode is compressively stressed (-); this type of bending occurs when the deposit is contracted and tends to increase in volume (8) The data

in Table 2 can be noticeably influenced by additives and this is discussed

in the chapter on Additives

Table 2: Stress Data for Some Electrodeposited Metals (1)

Deposit Melting Point ("Cy Stress(2)

MPa Cadmium

13.8 68.9 -3.4 to 10.3

*2000 -500 to 1500 2,ooo

10,000 20,000 40,000 60,000 60,000 100,000

1 From reference 2

2 Minus values represent compressive stress

Electrodeposits have been known to reduce the fatigue strength of

plated parts The reasons for this include: 1) hydrogen pickup resulting from

the cleaning/plating process, 2) surface tensile stresses in the deposits, and 3) lower strength of the deposits compared to the basis metal leading to cracks in the deposit which subsequently propagate through to the base metal A wealth of information on the influence of electrodeposits on fatigue strength can be found in reference 9 The discussion in this chapter will focus only on the influence of stress in electrodeposits on fatigue strength

A general rule of thumb is that tensile stresses in the deposits are deleterious, and the higher the stress the worse the situation in regards to fatigue strength of the substrate It is also important to realize that the strength of the steel also affects the amount of reduction in fatigue strength obtained after electrodeposition Data in Table 3 present information for a variety of deposits on two steels, SAE 8740 and SAE 4140 In all cases, a reduction in endurance limit was obtained as a function of increasing residual stress in the deposit

Table 3: Influence of Residual Stress in Various Electrodeposited Coatings on Fatigue Properties of SAE 8740 and SAE 4140 Steels SAE 8740 (AMs 6322) Steel (1)

Electrodeposit Deposit Residual Stress Endurance Limit ( 107cycles)

3 ,000 12,000 19,000

8 ,OOO

11,000 16,000 25,000 31,000

9 1,000 77,000 70,000 69,000 56,000

2 Data for 4140 are from reference 11 The tensile strength of the

steel was 1456 MPa (211,000 psi) Thickness of all deposits was 25

pm (1 mil)

Typically, chromium deposits highly stressed in tension reduce the fatigue strength of steel substrates to a greater degree than deposits with less stress (1 2-14) Compressively stressed chromium deposits reduce the fatigue strength of steel substrates very slightly or not at all, depending on the strength of the steel and degree of compressive stress

Shot peening before plating to induce compressive stress in the surface layers of the steel can help reduce the fatigue loss from subsequent plating Steel with a tensile strength of 1380 Mpa (200,000 psi) which had been reduced 47 percent in fatigue strength by chromium plating, was reduced only 10 percent in fatigue strength when it was shot peened before plating In another case, a steel with a tensile strength of 1100 MPa (1 60,000

psi) reduced 40 percent in fatigue strength by chromium plating was reduced only about 5 percent in fatigue strength when it was shot peened before plating (12) The federal chromium plating specification, QQ-C-320 calls for parts that are designed for unlimited life under dynamic loads to be shot peened and baked at 190°C ( 375°F) for not less than three hours

HOW TO MINIMIZE STRESS IN DEPOSITS

There are a variety of steps that can be taken to minimize stress in deposits:

A curve showing the relationship of stress in nickel deposited on different copper substrates is shown in Figure 2 The initial high stress is due

to lattice misfit and grain size of the underlying metal With fine grained substrates, the maximum stress is higher and occurs very close to the inter-

face As the thickness increases the stress decreases to a steady state value, the finer the substrate grain size, the more rapid this descent(2) The influence of the substrate on stress is also shown in Figure 3 which is a plot

of stress in electroless nickel coatings on a variety of substrates (aluminum, titanium, steel, brass and titanium) as a function of phosphorus content Be- sides showing that the substrate has a very distinct influence on stress due

to lattice and coefficient of thermal expansion mismatches, Figure 3 also shows that for each substrate a deposit with zero stress can be obtained by controlling the amount of phosphorus in the deposit (16)

Figure 2: Effect of grain size and deposit thickness on tensile stress in nickel deposited from a sulfamate solution at room temperature From reference 2 Reprinted with permission of ASM International

In terms of adhesion, the ideal case which would provide a true atomic bond between the deposit and the substrate is that wherein there is epitaxy or isomorphism (continuation of structure) at the interface Although this often occurs in the initial stages of deposition, it can only remain throughout the coating when the atomic parameters of the deposit and the substrate are approximately the same Since the stress which develops at the beginning of the deposition process, is in actuality a measure of bond strength, poor bonding shows up significantly in stress determinations This

is shown in Figure 4 for a nickel deposit on poorly cleaned and properly cleaned 304 stainless steel The poorly cleaned substrate had been allowed

Figure 3: Stress in electroless nickel as a function of phosphorus content

for metals with a high expansion coefficient (aluminum and brass) and a low expansion coefficient (steel, beryllium and titanium) Adapted from reference

5

Figure 4: Effect of bond strength on residual stress for Watts nickel

deposits on 304 stainless steel From reference 17 Reprinted with permission of Metal Finishing

to dry in air prior to nickel plating so that any oxide film destroyed by the cleaning process could reform, while the properly cleaned substrate was not dried prior to immersion in the nickel plating solution The deposit on the poorly cleaned substrate separated with a light pull while that on the properly

cleaned substrate could not be removed even with the use of a knife blade Figure 4 shows that the poor bonding was reflected by low initial stress

values, unlike the typically high values seen when good adhesion was present ( 17)

Influence of Plating Solution

The type of anion in the plating solution can leave a marked influ- ence on residual stress as shown in Table 4 for nickel deposits produced in different solutions Sulfamate ion provides nickel deposits with the lowest stress, followed by bromide which also reduces pitting (2) Not all plating solutions offer this wide range of anions capable of providing acceptable deposits but this option should not be ignored when looking for a deposit with low stress

Table 4: Influence of Anion on Residual Stress

33 .Ooo

a These data are from reference 2 Deposit thickness was 25

pm (1 mil), temperature was 25"C, and current density 323

amp/sq dm All solutions contained 1 M nickel, 0.5 M

boric acid, pH was 4.0 and the substrate was copper

Influence of Additives

There are numerous additives, particularly organic, which have a marked effect on the stress produced in deposits In fact, additives are so important in influencing properties of deposits that an entire chapter is