Indentation and bending behavior of electroless Ni-P-Ti composite coatings on pipeline steel

Accepted Manuscript Indentation and bending behavior of electroless Ni-P-Ti composite coatings on pipeline steel

Chuhong Wang, Zoheir Farhat, George Jarjoura, Mohammad K. Hassan, Aboubakr M. Abdullah PII: DOI: Reference:

S0257-8972(17)31106-4 doi:10.1016/j.surfcoat.2017.10.074 SCT 22839

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

2 August 2017 20 October 2017 24 October 2017

Please cite this article as: Chuhong Wang, Zoheir Farhat, George Jarjoura, Mohammad K. Hassan, Aboubakr M. Abdullah , Indentation and bending behavior of electroless NiP-Ti composite coatings on pipeline steel. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/ j.surfcoat.2017.10.074

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ACCEPTED MANUSCRIPT Indentation and Bending Behavior of Electroless Ni-P-Ti Composite Coatings on Pipeline Steel Chuhong Wang1, Zoheir Farhat1, George Jarjoura1, Mohammad K. Hassan2 and Aboubakr M. Abdullah2

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Department of Mechanical Engineering, Dalhousie University

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Halifax, NS, Canada B3J 2X4

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Center for Advanced Materials, Qatar University

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P.O.BOX 2713, Doha, Qatar

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ACCEPTED MANUSCRIPT Abstract Electroless Ni-P coatings have been adopted in many industrial applications due to their excellent wear and corrosion resistance. The unique properties and autocatalytic nature suggest a potential usage as a pipeline inner coating in oil and gas industries. However, low toughness and erosion resistance limit the suitability of

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electroless Ni-P coatings as pipeline coatings. To improve the toughness of the Ni-P coating, nano-sized

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titanium particles were used to reinforce Ni-P coating in this study. The effect of titanium particles on Ni-P

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coatings and the effect of annealing on the structure and mechanical properties were studied. The morphology

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and structure of Ni-P-Ti coating were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Vickers hardness, three-point bending and

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Hertzian indentation tests were employed to characterize the mechanical properties of Ni-P-Ti coating. It is found that adding titanium did not change the semi-amorphous microstructure of as-deposited Ni-P coating.

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The micro-hardness of as-deposited and annealed Ni-P-Ti coating was found to be higher than that of Ni-P

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coating. Furthermore, the addition of titanium particles increases toughness of the as-deposited coating. Also, it is found and toughness increases with increasing annealing temperature. The annealing also has a significant

Keywords

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effect on the indentation behavior of the coatings.

Electroless Ni-P coating; Nano-titanium particles; Bend tests; Indentation; Acoustic emission

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ACCEPTED MANUSCRIPT 1. Introduction Pipelines are a critical infrastructure in oil and gas transportation. Pipelines are considered to be the safest and most cost-effective way in Canada for transporting large volumes of petroleum product. Oil and gas are transferred by pipelines from excavation to refineries, terminals and markets. However, the erosion-corrosion

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damage of pipelines is an increasing problem across the petroleum industry due to the severe erosive and

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corrosive internal environment of pipelines. The presence of CO2 gas in pipelines causes internal corrosion,

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while the interaction between the internal pipeline surface and solid particles carried in the oil results in a

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progressive mechanical material loss [1-3]. The synergistic effect of erosion and corrosion leads to extensive material loss to the pipeline. In addition to erosion-corrosion damage, dents and gauges also contribute greatly

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to pipeline failures [4]. Many methods have been proposed to protect pipelines from damage, with internal coatings proving to be the most effective method [5]. Epoxy, polymer tapes and numerous composite coatings

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are used or tested as inner coating in the industry [5-7]. However, each of these methods have limitations.

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the purpose of this study.

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Therefore, developing a new low-cost coating that provides high erosion-corrosion and indentation resistance is

Electroless nickel plating has been used in many industries for protection due to its superb adhesion and unique

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ability to coat metal components regardless of their size and shape [8, 9]. Electroless Ni-P coating, which has excellent wear and corrosion resistance, has been adopted for many industries, such as aerospace, automotive and food industries. It is well known that amorphous Ni-P coating transforms to nickel phosphide and FCC nickel at 300-350°C [8, 10]. Annealing electroless Ni-P coating at a temperature up to 350 ºC can further improve its hardness and wear resistance as a result of phase transition. The high strength and excellent corrosion and wear resistance make electroless Ni-P coating a potential candidate for the oil and gas industry. However, many researchers have confirmed the brittleness and low toughness of the as-deposited and annealed 3

ACCEPTED MANUSCRIPT electroless Ni-P coating [4, 11]. In erosion of brittle materials, brittle fracture in which material is removed from the surface due to surface cracks, is found to be the dominant mechanism [12]. In this case, toughness becomes the most relevant material property that determines the erosion resistance [12, 13]. Thus, adding reinforcement to improve the toughness of electroless Ni-P is required.

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The addition of other particles into the Ni-P matrix can further improve its properties. For example, Al 2O3,

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which is one of the most widely used additive, can improve the hardness and wear resistance of the electroless

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Ni-P coating, while electroless Ni–P–Si3N4 and Ni–P–CeO2 can improve the corrosion resistance of plain Ni-P

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coating [14-17]. TiNi alloys are well known for their shape memory, superelastic properties and superb wear and dent resistance [18, 19]. Superelastic TiNi alloy has the ability to accommodate large scale deformation

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without generating permanent damage [19, 20]. Adding TiNi particles is a possible way to improve the toughness and indentation resistance of Ni-P coating. However, the difficulty and high-cost of producing TiNi

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particles limits the usage of TiNi. Adding Ti particles into Ni-P coating and forming TiNi during annealing is an

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alternative route to improving the properties of Ni-P coating. Even though superelasticity, high damping capacity and excellent corrosion and erosion resistance of electroplated Ni coatings containing Ti particles has

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been reported by many researchers [21-23], no literature exists on electroless Ni-P-Ti coatings. Thus, studies on

essential.

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the mechanical properties and effect of annealing on microstructural changes of electroless Ni-P-Ti coatings are

To investigate the possibility of utilizing Ni-P-Ti coating in oil and gas industry, it is necessary to characterize its toughness and indentation behavior. Indentation behavior can help to predict coating failure. During Hertzian indentation, different cracks such as Hertzian, radial, median, and lateral cracks may form [24, 25]. The formation of coating cracks during indentation testing is a key factor limiting the lifetime of materials and coatings. Toughness is another key property related to erosion resistance. There are many methods developed 4

ACCEPTED MANUSCRIPT to measure the toughness of coatings such as nanoindentation, tensile, bend, and scratch testing [26, 27]. Compared to other methods, bend tests provide a simple method and sample preparation. In addition, other mechanical properties such as Young’s modulus and fracture strength can also be calculated from bend tests. In this paper, bend tests were employed to investigate the Young’s modulus and toughness. Indentation tests

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were used to investigate the fracture properties of electroless Ni-P-Ti coating. The effect of annealing

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temperature on the microstructure and indentation and bending behavior of electroless Ni-P-Ti coatings were

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also studied.

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ACCEPTED MANUSCRIPT 2. Experimental procedure 2.1 Coating preparation and characterization A commercial solution Nichem 2500 from Atotech Inc was used as the coating solution. The solution contains

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nickel sulfate as the main salt and sodium hypophosphite as the reducing agent, 1g of titanium particle was added to a liter of the plating solution. The titanium powder used in this study consists of spherical

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nano-particles with a diameter of 70 nm from US Research Nanomaterials Inc. The composite solution was

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first mixed using an ultrasonic disperser from Fisher Scientific for 20 min and then stirred at 300 rpm during

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plating using a magnetic stirring system.

API X100 steel was used as the substrate in this study. Specimens were first ground using 240, 320, 400 and

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600 grit silicon carbide abrasive papers, then polished using 3µm diamond paste. Following polishing, a

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pre-treatment process including alkaline cleaning and acid etching was conducted on each specimen (Figure

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1). The alkaline cleaning solution used consists of 50g/L sodium hydroxide, 30g/L sodium carbonate and 30g/L sodium phosphate. Specimens were cleaned in an alkaline solution at 80±5ºC for 5 minutes and then

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rinsed using deionized water. After the alkaline cleaning, specimens were etched using a 15% aqueous H2SO4 solution for 15s. After the pre-treatment, specimens were first hung in the plating solution for 30 mins to

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create a thin layer of electroless Ni-P coating. Following the Ni-P plating, the specimens were plated in 1L Ni-P-Ti solution for 10 hours at a temperature of 88±1ºC and a pH of 4.7±0.1. The stirring system was used during the entire coating process. After plating for 10 hrs, specimens were then annealed in a vacuum furnace (WEG®) at 400, 600 and 800ºC for 1 hour, with heating and cooling rates of 20°C/min.

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Figure 1 Flow chart of coating preparation procedure

After annealing, an optical 3D profilometer was employed to investigate the roughness of the coating. An

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X-ray diffraction system from Bruker was used to study the crystal structure of the Ni-P-Ti composite coating.

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Cu-Kα radiation with a wave length of 1.54 Å was employed and operated at 40 KV and 40 mA. Specimens were scanned through 2θ of 20º to 120º at a scan rate of 0.2°/s.

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Vickers hardness of the coatings was measured using a MicroMet hardness tester under a load of 100g for 30s. The Vickers hardness was taken from five different locations on each specimen and the hardness values were

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2.2 Bend test

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then averaged.

Three-point bending tests were performed on samples using a PASCO ME-8236 testing apparatus. The width and thickness of the API steel specimens were 19mm and 1.16mm, respectively. Figure 2 is a schematic of the three-point bending test set-up, where F is the load applied at the center, l is the support span length and b is the width of the specimen. The thickness of the substrate is H and the thickness of the coating is h. The load F and the associated displacements were automatically recorded during the test.

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Figure 2 Schematic of three-point bending set-up

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The Young’s modulus of the substrate and coating-substrate bilayer system can be calculated from [28, 29]:

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is the Young’s modulus of the substrate,

Equation 2

is the Young’s modulus of the coating-substrate

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where

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bilayer system and k is the slope of the initial straight part of the force-displacement curve. To separate the Young’s modulus of the coating from the bilayer system, Equation 3, derived by Rouzaud [28], can be used.

Here,

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Many researchers have shown that this method yields accurate results [29-31].

is the Young’s modulus of the coating,

Equation 3 is the force that initiates the first crack and

is the

force in the substrate corresponding to the same deflection in the coating. This equation is derived from the energy balance of the system (Equation 4):

Equation 4

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ACCEPTED MANUSCRIPT where U is the energy consumed during the bending of the bilayer system; coating and

is the energy of bending the

is the bending energy of the substrate. In addition, the fracture strength of the coating

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be calculated from:

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Equation 5 where F is the applied force that corresponds to the initiation of the first crack and I is the moment of inertia

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of the specimen. The bend strength of the bilayer system can be calculated by:

Equation 6

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where F’ is the applied force for each specimen at the same deflection.

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2.3 Indentation test

API X100 steel disks having a thickness of 6mm and a diameter of 16mm were coated and then annealed

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using different temperatures of 400, 600 and 800°C. Hertzian-type indentation tests were conducted on as-deposited and annealed specimens using a PASCO ME-8236 apparatus. The tester uses a spherical WC-Co

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indenter (composition: 6% Co, 94% WC) with a diameter of 1.59 mm. The Vickers hardness and Young’s modulus of the indenter given by the supplier are 1620 and 650 GPa, respectively. The indentation load

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employed was 1000N. After indentation, an optical microscope was used to observe the indentation impression and cracks.

2.4 Acoustic emission Crack types can accurately be identified using optical microscopy. However, to investigate the fracture process and detect crack propagation, an acoustic emission sensor is necessary. A 1283 USB AE Node AE sensor was used in this study. When cracks form, fracture energy is released. The sensor attached onto the 9

ACCEPTED MANUSCRIPT coating surface detects the released energy and records the acoustic emission waves emitted during crack formation. Different parameters such as amplitude, counts and energy, can be collected using the sensor. It is established from previous research that the acoustic emission energy, which is related to fracture energy, is the best parameter for investigating crack propagation [32-34]. Thus, acoustic emission energy spikes were

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collected and used to indicate crack formation in the coatings during indentation and bend tests.

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3. Results and discussion

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3.1 Surface morphology and coating characterization

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The surface morphology of Ni-P and Ni-P-Ti coatings is shown in Figure 3. The surface of the plain Ni-P coating is smoother and more uniform than the Ni-P-Ti coating. Moreover, agglomeration of nano-titanium

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particles was observed on the surface. As a result, the presence of titanium particles led to an increase in

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roughness. The surface roughness of Ni-P and Ni-P-Ti coating are found to be 4 and 12µm, respectively.

Figure 3 Surface morphology of (a) Ni-P coating and (b) Ni-P-Ti coating

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Figure 4 (a) SEM micrograph and (b) EDS map on cross-section of Ni-P-Ti coating

The cross-section of the Ni-P-Ti coating is shown in Figure 4 (a). Figure 4 (b) shows the EDS map of the

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as-deposited Ni-P-Ti coating. The light (green) color represents titanium and the dark phases are Ni-P matrix

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(red) and substrate (blue). Titanium particles are well distributed in the coating. The coating/substrate interface was found to be smooth and uniform, identical to plain electroless Ni-P observed in former studies

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[4]. However, coating delamination was observed during the cutting process, which suggests that the addition

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of titanium powder decreases the interface strength.

Inductively coupled plasma mass spectrometry (ICP) is used to characterize the composition of the electroless

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Ni-P-Ti coating and substrate. The Ni-P coating was found to have a medium phosphorus content of

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10.11wt%. For Ni-P-Ti coating, the titanium content was found to be 6.6 wt%. The composition of the API X100 pipeline steel substrates was also analyzed and the composition of the coatings and substrate are shown in Table 1.

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ACCEPTED MANUSCRIPT Table 1 Composition of substrate and coatings

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Ni

P

Cr

Si

Cu

Ti

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Mn

Fe

API X100

0.103

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0.010

0.070

0.121

0.009

0.018

0.036

1.221

Balance

Ni-P coating

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89.89

10.11

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Ni-P-Ti coating

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83.8

9.6

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---

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Composition (wt%)

6.6

---

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The microstructure of electroless Ni-P-Ti composite coatings was also studied. It is well known that

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electroless Ni-P coatings with high phosphorus content have a semi-amorphous microstructure. Furthermore,

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the semi-amorphous Ni-P phase transforms to crystalline nickel phosphide and FCC nickel at above 300°C [10, 35, 36]. To investigate the effect of titanium nano-particles on the microstructure of electroless Ni-P

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coating during annealing, the annealing temperatures in this study were selected above the crystallization

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coatings are shown in Figure 5.

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temperature, namely 400, 600 and 800 °C. The XRD patterns of the as-deposited and annealed Ni-P-Ti

The as-deposited Ni-P-Ti coating has a broad amorphous peak at the 2θ position of 45º. Titanium peaks were

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not identified in the XRD pattern of the as-deposited Ni-P-Ti, which may be due to the low amount and the fine nano–size of titanium particles. It is well known that nano-particles tend to have broad XRD peaks.

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Moreover, the main peak of titanium is at the 2θ position of 40º, which overlaps with the peak of amorphous Ni-P coating. It is clear that annealing results in the crystallization of Ni and precipitation of Ni3P. Similar results have been obtained in previous XRD analysis of plain Ni-P coating in earlier work [37]. Furthermore, at high annealing temperature, the presence of Ni3Ti was identified. Meanwhile, the addition of titanium particles affected the percentage of FCC nickel phase in Ni-P-Ti coating. It has been confirmed by several researchers [35, 38, 39] that the amount of nickel and nickel phosphide vary with temperature. Keong [38]

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ACCEPTED MANUSCRIPT indicated that at low annealing temperatures under 500°C, the crystallization is not completed and there is still an amorphous phase in the Ni-P coating. The crystallization ends at 600-800°C and at 800°C the coating is completely crystallized. Therefore, in plain Ni-P coating, the percentage of crystalline phase increases with an increasing annealing temperature. Nevertheless, in Ni-P-Ti coating, there is no significant change in

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intensities of nickel and Ni3P with the increasing temperature. The existence of the titanium particles results in

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the formation of Ni3Ti during annealing and reduces the amount of crystallization nickel in high temperature

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annealed coatings.

Figure 5 XRD patterns of as-deposited electroless Ni-P and Ni-P-Ti coatings under different annealing temperatures

Vickers hardness of both as-deposited and annealed Ni-P-Ti coating is shown in Figure 6. Previous research confirms that annealing increases the hardness of Ni-P coatings due to phase transition [9, 17, 40]. With an increasing annealing temperature, the percentage of nickel phase in the coating increases, as well as the grain 13

ACCEPTED MANUSCRIPT size, resulting in the decrease in hardness. The as-deposited coating, which has an amorphous structure, has a relatively low hardness. Ni-P coating annealed at 400°C was found to have the highest hardness. The effect of annealing on hardness of Ni-P-Ti coatings follows the same trend as that of annealed Ni-P coatings [37]. The as-deposited Ni-P-Ti coating has the lowest hardness of 6.68 GPa (HV 681). It is higher than that of Ni-P

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coating, which according to research literature, has an HV of 410-600 [9, 17, 41]. The 400°C annealed

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Ni-P-Ti has the highest hardness of 12.63 (HV 1287) GPa. It is also higher than the 400°C annealed Ni-P

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coating, which was found to have a Vickers hardness between 1000-1100 in literature. Similar to the Ni-P coating, the hardness of annealed Ni-P-Ti coating drops with an increasing annealing temperature. The

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increasing percentage of crystalline phases and the growth of the grain size may contribute greatly to the drop

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in the hardness with the increasing annealing temperature.

Figure 6 Hardness of Ni-P based coatings as a function of annealing temperature

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ACCEPTED MANUSCRIPT 3.2 Bending behavior of electroless Ni-P coating The bending behavior of as-deposited and annealed Ni-P-Ti coatings was investigated using three-point bending tests. To study the effect of titanium powder on the bending behavior of the Ni-P coating, plain as-deposited Ni-P coating was also tested. Acoustic emission energy changes during bending and these

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changes were detected using an acoustic emission detection system. The acoustic emission sensor collects an

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energy spike during the initiation of a crack. The force initiating the first crack can be measured by

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superimposing the force-displacement and acoustic emission curves. Figure 7 shows the force-displacement curves and acoustic emission data of substrate and as-deposited Ni-P and Ni-P-Ti coatings. Compared to the

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steel substrate, it is evident that both as-deposited Ni-P and Ni-P-Ti coatings increase the required force to

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induce a given deflection.

In Figure 7, a localized drop in bending force can be observed in both Ni-P and Ni-P-Ti coating. This drop is

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associated with acoustic energy spikes, providing evidence of crack formation. It can be observed that both Ni-P and Ni-P-Ti coating have similar elastic behavior, but the required force for crack initiation for Ni-P-Ti is higher than that of Ni-P coating. In addition, the deflection of Ni-P-Ti coatings (where the crack happens),

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is higher than that of Ni-P coatings as well.

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Figure 7 Force-displacement and acoustic emission data collected during bend tests of as-deposited coatings

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and substrate

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Figure 8 Force-displacement and fracture force associated with acoustic emission of bend tests on Ni-P-Ti

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coatings and substrate

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The effect of annealing temperature on the bending behavior of Ni-P-Ti coating is evaluated similarly. Figure 8 illustrates the load–displacement curves of as-deposited and annealed Ni-P-Ti coating recorded through

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three-point bending tests. The force that initiated the first crack (initial fracture force) was measured with the

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help of the acoustic emission energy changes, as shown in Figure 8.

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Figure 9 Cracks after bend tests on (a) (c) as-deposited Ni-P-Ti and (b) (d)) 800 ºC annealed Ni-P-Ti

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coatings

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From Figure 8, it can be observed that only as-deposited coating exhibits a sudden drop in the bending force after generation of cracks. To investigate the fracture morphology of as-deposited and annealed Ni-P-Ti

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coating, specimens were observed using an optical microscope. A large crack and coating delamination can be

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observed on the as-deposited coating (Figure 9 (a)). From cross-sectional view of the bending specimen,

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coating delamination was observed on as-deposited coating (Figure 9 (c)). The delamination is associated with the drop of bending force in force-displacement curves in Figure 8. In comparison to the as-deposited coating,

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no large cracks were observed on the annealed coating and some fine cracks formed on the bending specimens (Figure 9 (b)(d)). The crack morphology of Ni-P-Ti coating indicates that diffusion may have occurred on the

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interface during annealing, and resulted in increasing interface strength of the Ni-P-Ti coating.

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Figure 10 Bend strength of steel, Ni-P and Ni-P-Ti coatings as a function of annealing temperature

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To study the effect of Ni-P-Ti coatings on the bending behavior of the steel substrate, the bend strengths of the

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substrate and coating/substrate bilayer systems (at a deflection of 1.5 mm) were calculated using Equation 6, shown in Figure 10. It can be observed that the as-deposited Ni-P-Ti coating increases the bend strength of the

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steel. However, the annealed coatings exhibit a drop in bend strength, which indicates that annealed coatings are less capable of supporting the steel substrate during bending. The bend strength of the as-deposited coating

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is due to its higher toughness. The formation of nickel phosphide phase during annealing results in the drop of toughness and causes the drop in bend strength, which is discussed next.

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Figure 11 Young’s modulus of Ni-P coating and Ni-P-Ti coatings

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The slope of the elastic deformation in the force-displacement curve (Figure 8) is related to the Young’s

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modulus. The Young’s moduli of as-deposited Ni-P and as-deposited and annealed Ni-P-Ti coating are shown in Figure 11. The Young’s moduli of the as-deposited Ni-P and Ni-P-Ti coatings are found to be similar. On

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the other hand, the effect of the annealing on the Young’s modulus is significant. Specimens annealed at 400

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and 600ºC exhibit a higher Young’s modulus than as-deposited coatings due to the crystallization post-annealing. Generally, amorphous or semi-amorphous metallic materials exhibit lower Young’s modulus compared with crystalline metals [42, 43]. Crystallization during annealing is attributed to the partial removal of the plastic instabilities in as-deposited Ni-P and Ni-P-Ti coatings [17]. The Young’s modulus appears to decrease at an annealing temperature of 800°C due to the formation of Ni3Ti.

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Figure 12 Toughness, fracture force and hardness of Ni-P based coatings

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Toughness, which is the ability of a material to absorb energy before fracture, is also measured to

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characterized the coatings. The toughness of Ni-P and as-deposited and annealed Ni-P-Ti coatings were calculated from the force-displacement curves (Figure 8), calculating the area under the curve from the start of

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the bend test until the first crack initiation. The force that initiates the first crack is defined as fracture force. As mentioned above, the fracture forces of annealed coatings are lower than those of as-deposited coatings.

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Figure 12 shows the relationship of toughness, fracture force and hardness for both Ni-P and Ni-P-Ti coating. Comparing as-deposited Ni-P with Ni-P-Ti coating, it is found that the addition of titanium particles in the coating increases the hardness, fracture force and toughness. It is evident that the fracture force is highly dependent on the hardness and microstructure of Ni-P and Ni-P-Ti coatings. Both as-deposited Ni-P and Ni-P-Ti coatings have higher toughness compared to the annealed coatings due to the semi-amorphous structure of as deposited coating. The trend of variation across initial fracture force, as a function of annealing

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ACCEPTED MANUSCRIPT temperatures, is opposite to that of hardness, as shown in Figure 12. The 400°C annealed specimen, which has the highest hardness, has the lowest initial cracking force and toughness. These results suggest that annealing increases brittleness of Ni-P coating, with the 400°C annealed coating being the most brittle coating.

The relationship between the hardness and toughness of Ni-P and Ni-P-Ti coatings are related to the

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microstructure transitions of the coatings during annealing. The semi-amorphous microstructure of

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as-deposited Ni-P and Ni-P-Ti coating is characterized by low hardness and high toughness. In the 400°C

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annealed coating, nickel phosphide is the dominant phase. The nickel phosphide peak is broad when compared

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to coatings annealed at higher temperatures in an XRD pattern, which indicates that a 400°C annealed coating has relatively fine grain sizes. This coating has a high hardness, but lower toughness. Coatings annealed at

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temperatures of 600 and 800°C contain nickel, Ni3P and Ni3Ti and they are well crystalized. The crystallized

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structure causes the drop in hardness, but an increase in toughness.

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Figure 13 Fracture strength of Ni-P-Ti coatings as a function of annealing temperature

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Fracture strength, also known as flexural stress,, defined as the stress at which the specimen fracture, is a commonly used property to study brittle materials and it is often characterized by a large variability [44].

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Fracture strength is sensitive to size, shape, loading rate and test environment [45]. Fracture strength

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calculated using Equation 5 is shown in Figure 13. The force used to calculate the fracture strength is the initial fracture force. The effect of the annealing temperature on fracture strength can be explained by the change in the microstructure of the coating as well. The as-deposited coating was found to exhibit high fracture strength due to the high toughness of its amorphous structure. The coatings annealed at 400ºC are the most brittle and have the lowest fracture strength due to the nano-crystalline structure. High temperature annealed coatings which are fully crystallized with Ni3Ti particles, have higher toughness.

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ACCEPTED MANUSCRIPT In summary, the effect of titanium particles on mechanical properties of as-deposited coatings and the effect of annealing temperature on mechanical properties of Ni-P-Ti coatings were studied. The results are

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summarized in

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ACCEPTED MANUSCRIPT Table 2. Comparing the as-deposited Ni-P and as-deposited Ni-P-Ti coatings reveals that the addition of the titanium can increase the hardness, toughness, fracture strength of the coatings and bend strength of the bilayer system. However, it decreases the Young’s modulus. The effect of annealing temperature is also

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shown in

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ACCEPTED MANUSCRIPT Table 2. As mentioned above, with an increasing temperature, the percentage of crystalline phases in the coating increases, as well as the grain size. The change in microstructure causes a drop in the hardness, but it increases the toughness, fracture strength of the coatings Young’s modulus of Ni-P-Ti coatings also increases with the annealing temperature. In general, annealed Ni-P-Ti coatings have higher hardness, higher Young’s

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modulus and lower toughness than that of as-deposited Ni-P-Ti coatings due to the semi-amorphous

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microstructure of as-deposited coatings. The superelastic TiNi phase was not detected by XRD; however, this

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might be due to the fact that only small amounts of TiNi have formed. Therefore, the improvement in

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needed to confirm the presence of superelastic TiNi.

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toughness of annealed Ni-P-Ti coatings might be due, at least in part, to superelastic behavior. Further work is

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Young’s Hardness

Bend strength of the

Strength

bilayer system

↑

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↓

Toughness

↑

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Modulus The addition of

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Table 2 Effect of titanium particles and annealing temperature

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annealing

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temperature

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ACCEPTED MANUSCRIPT 3.3 Indentation behavior of electroless Ni-P-Ti coatings The indentation morphology as well as the load-displacement of as-deposited annealed electroless Ni-P-Ti coatings are summarized in Figure 14. The acoustic emission associated with the crack propagation is also shown in Figure 14. Red dots represent energy change during loading while blue triangles show that of

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unloading.

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Hertzian and radial cracks are found on as-deposited coating. The radial cracks may be caused by residual

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stresses in accordance with the expanding cavity model [46]. Similar to bending fracture, the as-deposited

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coating has a low density of radial cracks, however, the cracks are larger in size. Moreover, coating delamination is identified next to a radial crack. According to the acoustic emission data, cracks that appeared

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on the as-deposited coating released more energy than cracks on the annealed coatings. Interestingly, acoustic emission energy spikes were also recorded during unloading of the as-deposited Ni-P-Ti coating, which is not

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evident on annealed coatings. This means more energy was absorbed and stored during loading because of the high toughness of Ni-P-Ti coating. And the residual stresses during loading is released during unloading and causes the propagation of radial cracks during unloading. In the previous study [4], it was found that there

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were no Hertzian cracks observed on as deposited Ni-P coating under load of 2000N and only micro-bend

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cracks were observed around the contact edge. For electroless Ni-P-Ti coating, the first crack was observed at 300 N. The agglomeration of titanium particles and the low bond between titanium particles and Ni-P matrix may results in premature localized failure during indentation.

Coatings annealed at temperatures of 400 and 600°C appears to have a high density of micro-cracks. According to the acoustic emission data, the sensor detected crack formation at a low load under 100N and energy spikes were detected during the loading process. The acoustic emission energy spikes are lower

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ACCEPTED MANUSCRIPT compared to as-deposited coating. During unloading, there were no cracks generated. As discussed above, the specimen annealed at 400°C was found to have the highest hardness and lowest toughness, which explain the low indentation resistance of the annealed coating.

The load-displacement curve and indentation views of the coating annealed at 800°C are shown in Figure 14

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(d). The coating annealed at 800°C shows Hertzian and radial cracks of lower density compared to the

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coatings annealed at 400 and 600°C. However, ring cracks away from the contact edge were also found on the

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surface. According to the acoustic emission data, an energy spike associated with the initiating crack force

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was found at a load of 394N. Additionally, the spike released more energy than the energy spikes of the 400 and 600°C annealed coatings.

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In summary, the effect of titanium particles on the indentation behavior of as-deposited Ni-P coating can be evaluated by comparing this study’s results with the previous study [4]. Radial cracks and coating

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delamination were observed on Ni-P-Ti coating, while only micro-bend cracks were observed on Ni-P coating under a load of 1500N. Data from bend tests has indicated that adding titanium particles increases the bend

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strength and toughness in general. However, the agglomeration of titanium particles and the low bond between titanium particles and Ni-P matrix may results in premature localized failure during indentation. The

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effect of annealing on the indentation behavior of Ni-P-Ti coating was also studied. Annealing significantly changes the indentation behavior of Ni-P-Ti coating. The as-deposited coating has the lowest hardness and the highest toughness. Annealing increases the hardness and lowers the toughness. The annealed coatings become more brittle due to phase transition. The coatings annealed at 400 and 600°C began to crack under a load of 100N and showed a large density of micro-cracks. For coatings annealed at high temperatures, the changed percentage of crystalline nickel and nickel phosphide phase caused the decrease in hardness and increase in toughness. Hence, the dent resistance of the coating increases and crack density drops. 29

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Figure 14 Indentation fracture and load-displacement curve of the as-deposited and annealed specimens

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ACCEPTED MANUSCRIPT 4. Conclusion In this study, electroless Ni-P-Ti coatings were deposited on API X100 steel and the specimens were then annealed at different temperatures. The morphology, hardness, bending and indentation behavior of electroless Ni-P coating were studied. The effects of annealing and the addition of titanium particles to Ni-P coating were

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investigated. The following conclusions can be drawn from the current study:

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1. As-deposited Ni-P-Ti coating has a semi-amorphous structure similar to Ni-P coating. Crystalline

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nickel and nickel phosphide phases are identified in Ni-P-Ti coating after annealing at 400°C, Ni3Ti is

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observed in Ni-P-Ti coating at 600 and 800 °C annealing.

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2. The addition of titanium particles increases the hardness of as deposited and annealed electroless Ni-P coating. Coating annealed at 400°C exhibits the highest hardness, followed by a decreasing hardness

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as annealing temperature continues to increase.

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3. The Young’s modulus and toughness of annealed coatings were investigated through bend tests. The addition of titanium particles increases toughness and decreases Young’s modulus, while both

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toughness and Young’s modulus increase with increasing annealing temperature. Coating annealed at

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400°C is found to have the lowest toughness.

4. Hertzian cracks and radial cracks were observed on the as-deposited and coatings annealed at low temperatures. For coatings annealed at high temperatures, Hertzian, ring and radial cracks were found on the surface. However, annealing increases the interface strength between coatings and substrates. Through bending and indentation fracturing of as-deposited coating, coating delamination was identified

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ACCEPTED MANUSCRIPT 5. Acoustic emission technique was found to be useful in detecting crack initiation during bending and indentation testing. The initiation force of the first crack can be determined using the acoustic emission technique. Acoustic emission is instrumental for identifying cracks during both loading and

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unloading.

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ACCEPTED MANUSCRIPT Acknowledgement This publication was made possible by NPRP grant #NPRP8‐1212‐2‐499 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of

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ACCEPTED MANUSCRIPT Highlights 1. Young’s modulus and toughness of electroless Ni-P-Ti composite coatings were measured using three point bending tests.

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2. Indentation behavior of Ni-P-Ti coatings was identified and related to the microstructure of

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coatings.

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3. Acoustic emission is found to be useful in detecting crack formation in Ni-P-Ti composite

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coatings.

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