The Effect of Crystallization and Phase Transformation on the Mechanical and Electrochemical Corrosion Properties of Ni-P Coatings

coa ings A icle The E ec o C ys alliza ion and Phase T ans o ma ion on he Mechanical and Elec ochemical Co osion P ope ies o Ni-P Coa ings Ma in Buch ík1,* , Leoš Doskoˇcil 1, Roman B esche 1, Pa el Doležal 1,2 , Jiˇ íMásilko 1and Ja omí Wasse baue 1   Ci a ion: Buch ík, M.; Doskoˇcil, L.; B esche , R.; Doležal, P.; Másilko, J.; Wasse baue , J. The E ec o C ys alliza ion and Phase T ans o ma ion on he Mechanical and Elec ochemical Co osion P ope ies o Ni-P Coa ings. Coa ings 2021,11, 447. h ps://doi.o g/ 10.3390/coa ings11040447 Academic Edi o : Ke in Pluckne Recei ed: 2 Ma ch 2021 Accep ed: 7 Ap il 2021 Published: 13 Ap il 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). 1Ma e ials Resea ch Cen e, Facul y o Chemis y, B no Uni e si y o Technology, Pu kyˇno a 464/118, 612 00 B no, Czech Republic; [email p o ec ed] (L.D.); [email p o ec ed] (R.B.); [email p o ec ed].cz (P.D.); [email p o ec ed] (J.M.); [email p o ec ed] (J.W.) 2Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, Technická2896/2, 602 00 B no, Czech Republic *Co espondence: [email p o ec ed]; Tel.: +42-(073)-644-5019 Abs ac : This pape deals wi h he s udy o he c ys alliza ion and phase ans o ma ion o Ni-P coa ings deposi ed on AZ91 magnesium alloy. P epa ed samples we e cha ac e ized in e ms o su ace mo phology and elemen al composi ion by means o scanning elec on mic oscopy wi h ene gy-dispe si e spec oscopy analysis. The esul s o X- ay di ac ion analysis and di e en ial scanning calo ime y sugges ed ha inc easing he phospho us con en caused Ni-P coa ings o de elop an amo phous cha ac e . The c ys alliza ion o Ni was obse ed a 150, 250, and 300 ◦ C o low-, medium- and high-phospho us coa ings, espec i ely. The Ni c ys alli e size inc eased wi h inc easing empe a u e and dec easing P con en . Con e sely, he p esence o he Ni 3 P phase was obse ed a a maximum peak o 320 ◦ C o he high-phospho us coa ing, whe eas he c ys alliza ion o he Ni 3 P phase shi ed o highe empe a u es wi h dec easing P con en . The Ni 3 P c ys alli e size inc eased wi h inc easing empe a u e and inc easing P con en . An inc ease in mic oha dness due o he a angemen o Ni a oms and Ni 3 P p ecipi a ion was obse ed. The deposi ion o as-deposi ed Ni-P coa ings led o an imp o emen in he co osion esis ance o AZ91. Howe e , he hea ea men o coa ings esul ed in a de e io a ion in co osion p ope ies due o he o ma ion o mic oc acks. Keywo ds: Ni-P coa ing; AZ91 alloy; phase ans o ma ion; c ys alliza ion; co osion beha io 1. In oduc ion Magnesium and i s alloys ha e unique p ope ies such as low densi y, a high s eng h o weigh a io, and good cas abili y, and a e hus o g ea in e es in many a eas o indus- y, especially in he au omo i e indus y, a ia ion, and elec ochemis y [ 1 – 3 ]. Howe e , poo co osion esis ance, low ha dness, and low wea esis ance a e hei main disad an- ages [ 3 , 4 ]. One way o p o ec ing magnesium and i s alloys is he applica ion o coa ings. Elec oless Ni-P coa ings deposi ed on magnesium alloys ha e g ea po en ial o many indus ial applica ions [ 4 , 5 ]. Fo example, hey can imp o e esis ance agains ex e nal in luences, wea , o co osion, and can also con ibu e o imp o ing he appea ance o he coa ed pa [6–9]. The p ope ies o elec oless Ni-P coa ings depend on hei phospho us con en and he hea ing p ocess, and, hus, on hei mic os uc u e [ 10 , 11 ]. I was epo ed [ 6 , 12 ] ha he ha dness and wea esis ance o Ni-P coa ings dec ease wi h inc easing P con en . On he o he hand, co osion esis ance should inc ease wi h inc easing P con en , as shown in he wo ks o Mainie [13] and Aga wala [14]. I is well known ha elec oless Ni-P coa ings can be dis inguished on he basis o whe he hey a e low-phospho us (LP) Ni-P coa ings (app oxima ely 1–5 w . % o P), medium- phospho us (MP) Ni-P coa ings (6–9 w . % o P), o high-phospho us (HP) Ni-P coa ings Coa ings 2021,11, 447. h ps://doi.o g/10.3390/coa ings11040447 h ps://www.mdpi.com/jou nal/coa ings Coa ings 2021,11, 447 2 o 15 (10–13 w . % o P) [ 12 ]. LP Ni-P coa ings a e c ys alline o mic oc ys alline, which indica es ha he amoun o phospho us a oms in in e s i ial posi ions is no su icien o dis o he nickel la ice [ 15 – 17 ]. MP Ni-P coa ings a e o med by bo h a mix u e o mic oc ys alline nickel and an amo phous phase, wi h he ac ion o he amo phous phase inc easing wi h in- c easing phospho us con en in he coa ing. HP Ni-P coa ings a e cha ac e ized as comple ely amo phous [ 18 , 19 ]. The amo phous phase is p esen in he coa ing because he solubili y o phospho us in nickel is e y low (>0.17 % o P) [ 6 , 20 ]. The amo phous phase egions g ow due o an inc ease in la ice dis o ion caused by phospho us a oms becoming si ua ed in he in e s i ial posi ions o he nickel la ice. As-deposi ed Ni-P coa ings a e he modynamically uns able and he coa ing ends o become a he modynamically mo e s able and ene gy-e icien equilib ium s a e [ 11 , 20 , 21 ]. Acco ding o some au ho s, unde equilib ium condi ions, below he mel ing poin o Ni-P coa ings (880 ◦ C), only wo phases should be p esen in he Ni-P coa ing—an α phase o med by less han 0.17 w . % o P dissol ed in Ni and an in e media e nickel phosphide Ni 3 P phase con aining 15 w . % o P [ 6 , 12 , 22 ]. Howe e , he equilib ium phase diag am can only be used o desc ibe he mic os uc u e o alloys in he equilib ium s a e, e.g., a e he hea ea men . To cla i y he desc ip ion o Ni-P coa ings deposi ed om he elec oless pla ing ba h, i is necessa y o use a nonequilib ium phase diag am. Acco ding o Riedel’s s udy [ 12 ], he mic os uc u e o N-P coa ings can be ans- o med du ing hea ea men , esul ing in changes in he a omic s uc u e. Bo h he mic oc ys alline and amo phous phases unde go he c ys alliza ion p ocess, and e ag- onal in e media e phase Ni 3 P is o med a he same ime [ 11 ]. Apachi ei e al. [ 23 ] and Duncan [ 11 ] epo ed ha he me as able β phase and he amo phous γ phase a e subjec o decomposi ion eac ions, whe e he s able c ys alline α phase and Ni 3 P a e o med. Some au ho s [ 16 , 17 , 24 ] epo ed ha MP and HP coa ings can be o med by mix u es o mic oc ys alline nickel and a ious c ys alline nonequilib ium phases such as Ni 5 P 4 , Ni 12 P 5 , and Ni 5 P 2 c ea ed du ing hea ea men up o 300 ◦ C. Abo e his empe a u e, he Ni3P phase is o med om hese me as able nonequilib ium phases. This s udy is ocused on he c ys alliza ion and phase ans o ma ion o deposi ed Ni-P coa ings wi h a ious phospho us con en s and a emp s comp ehensi ely o de- sc ibe he ela ionship be ween phospho us con en and he mic os uc u al, mechanical, and elec ochemical co osion p ope ies o Ni-P coa ings deposi ed on Mg alloy AZ91. The mic os uc u e and phase ans o ma ions o deposi ed Ni-P coa ings we e de e mined du ing a con inuous hea ing p ocess using X- ay di ac ome y (XRD) and di e en ial scanning calo ime y (DSC). Subsequen ly, he mic oha dness and elec ochemical co o- sion p ope ies o he coa ings we e de e mined. The esul s a e discussed in e ms o he obse ed mic os uc u al changes and phase ans o ma ions. 2. Ma e ials and Me hods Samples o AZ9-cas magnesium alloy wi h dimensions o 30 mm × 30 mm × 7 mm we e used as subs a es o he elec oless deposi ion o Ni-P coa ings. The chemical composi ion o he AZ91 alloy is lis ed in Table 1. The elemen al analysis o he magnesium subs a e was pe o med using glow-discha ge op ical emission spec oscopy (GDOES) on a Spec uma GDS 750 ins umen (Spec uma Analy ik GmbH, Ho , Ge many). Table 1. Elemen al composi ion o AZ91 magnesium alloy, glow-discha ge op ical emission spec- oscopy (GDOES); Mg balance. Elemen Al Zn Mn Si Fe Z Con en (w . %) 8.80 0.81 0.32 0.01 0.004 0.01 The samples o AZ91 alloy wi h deposi ed Ni-P coa ings we e p epa ed in he same way as epo ed in p e ious wo k [ 25 ]. The Ni 2+ /H 2 PO 2− a ios in he elec oless nickel ba h we e adjus ed in o de o deposi Ni-P coa ings wi h a high, medium, and low P Coa ings 2021,11, 447 3 o 15 con en . The deposi ion ime was 4 h and he a e age hickness o he coa ings was app oxima ely 30 µ m. The hea ea men o Ni-P coa ings ( o he de e mina ion o mic oha dness and elec ochemical co osion p ope ies) was pe o med in a LAC LM07 mu le u nace (LAC, s. .o., Židlocho ice, Czech Republic) a 400 ◦C o 1 h. The mo phology and elemen al composi ion o he deposi ed Ni-P coa ings we e analyzed using a Zeiss EVO LS-10 scanning elec on mic oscope (SEM) (Ca l Zeiss L d., Camb idge, UK) wi h ene gy-dispe si e spec oscopy (EDS), an Ox o d Ins umen s Xmax 80 mm 2 de ec o (Ox o d Ins umen s plc, Abingdon, UK), and AZ ec so wa e ( e sion 2.4, Ox o d Ins umen s, High Wycombe, UK). The deposi ed Ni-P coa ings we e mechanically sepa a ed om he magnesium sub- s a es and c ushed o a ine powde in an aga e mo a . The phase analysis o he Ni-P powde was pe o med on a P pan using an Empy ean X- ay di ac ion (XRD) spec ome- e (PANaly ical, Mal e n, UK) wi h a high- empe a u e chambe (An on Paa HTK 16N, An on Paa , G az, Aus ia). The pa ame e se ings we e as ollows: Cu K α adia ion ( λ K α 1 = 0.15406 nm, λ K α 2 = 0.15444 nm); scan ange om 25 o 65 ◦ ; scan s ep size, 0.013 ◦ 2 θ ; ime pe s ep, 39 s; gene a o ol age, 40 kV; and ube cu en , 30 mA. X- ay di ac ion pa e ns we e eco ded in he empe a u e ange om 50 o 550 ◦ C wi h a pa e n eco d s ep o 50 ◦C. To achie e a mo e accu a e eco d o phase changes, he pa e n eco d s ep was se o 10 ◦C o he empe a u e ange om 300 o 400 ◦C. The c ys alli e size o he Ni and Ni 3 P phases was calcula ed om he ull wid h hal maximum (FWHM) acco ding o he Sche e equa ion using HighSco e Plus ( e sion 3.0.5, PANaly ical B.V., Almelo, The Ne he lands) so wa e (Equa ion (1)): τ= K×λ β1/2 ×cosθ, (1) whe e τ is he c ys alli e size [Å], λ is he X- ay wa eleng h (nm), β1/2 is he peak ex ension a hal he maximum in ensi y (FWHM), θ is he di ac ion B agg’s angle, and K is he shape ac o (Sche e cons an ) anging om 0.62 o 2.08 (usually close o 1). To unde s and he c ys alliza ion and ans o ma ion beha io o Ni-P coa ings, di - e en ial scanning calo ime y (DSC) analysis was pe o med. Ni-P coa ings, sepa a ed and c ushed o powde , we e con inually hea ed a a hea ing a e o 10 ◦ C · min −1 om 50 o 500 ◦ C using a DSC F1 204 di e en ial scanning calo ime e (Ne zsch, Selb, Ge many). Samples o abou 10 mg we e placed in o Al pans. An emp y pan was used as a e e ence. The mic oha dness o Ni-P coa ings was measu ed using an LECO AMH55 (Leco, Sain Joseph, MO, USA) Vicke s mic oha dness es e unde an applied load o 25 g and wi h a dwell ime o 10 s. The mic oha dness was measu ed en imes on a polished coa ed-sample c oss-sec ion. The elec ochemical co osion p ope ies o as-deposi ed and hea - ea ed Ni-P coa - ings we e analyzed by means o a po en iodynamic pola iza ion es in 3.5% NaCl solu ion using a Bio-Logic VSP-300 po en ios a /gal anos a (BioLogic, Seyssine -Pa ise , F ance) a oom empe a u e. The solu ion was boiled be o e po en iodynamic measu emen o emo e CO 2 and oxygen. In he case o plain AZ91 magnesium alloy, he specimen su - ace was g ound using SiC pape #1200. I was hen cleaned wi h dis illed wa e and isop opanol and d ied wi h a s eam o d y ai . Immedia ely a e wa d, he measu emen was pe o med. In he case o coa ed magnesium alloy, he samples we e always cleaned be o e he measu emen wi h dis illed wa e and isop opanol and d ied wi h a s eam o d y ai . The analyzed a ea o samples was app oxima ely 1 cm 2 . The measu emen was pe o med using a s anda d h ee-elec ode cell: P gauze was used as a coun e -elec ode, a sa u a ed calomel elec ode (SCE) as a e e ence elec ode, and a p epa ed sample as a wo king elec ode. The po en ial ange was se om − 200 mV o +250 mV s. open ci cui po en ial (OCP) and he scan a e was 1 mV · s −1 . The s abiliza ion ime o he samples exposed o he co osi e en i onmen was 10 min. Values o co osion po en ial Eco and co osion cu en densi y ico we e de e mined by applying he Ta el analysis. Coa ings 2021,11, 447 4 o 15 3. Resul s and Discussion 3.1. Mo phology o Deposi ed Ni-P Coa ings The su ace mo phology o Ni-P coa ings deposi ed on AZ91 magnesium alloy is shown in Figu e 1. The deposi ed Ni-P coa ings had high—(10.8 ± 0.1 w . % o P), medium—(7.4 ± 0.1 w . % o P), and low—(5.5 ± 0.1 w . % o P) phospho us con en s. All deposi ed coa ings show a nodula mo phology, which is ypical o elec oless Ni- P coa ings [ 12 ]. As obse ed in Figu e 1a–c, he nodule size dec eases wi h inc easing P con en . As epo ed by Shu [ 26 ], he phospho us has e y low solubili y in nickel; he e o e, he educed phospho us ends o agg ega e a he bounda ies o he nickel g ains du ing deposi ion. Hence, he phospho us inhibi s he g ow h o Ni pa icles and pa icles o Ni-P and inc eases he numbe o nuclea ion si es. The same dependence o nodule size on phospho us con en was also obse ed by Ash iani e al. [ 10 ]. Some s uc u al de ec s and associa ed c acks we e a ely obse ed in MP Ni-P coa ings. These mic os uc u al de ec s (g ow hs, mic opo es, mic oc acks) inc eased he oughness and wo sened he co osion esis ance o MP Ni-P coa ings. Coa ings 2021, 11, x FOR PEER REVIEW 4 o 15 po en ial (OCP) and he scan a e was 1 mV·s −1 . The s abiliza ion ime o he samples exposed o he co osi e en i onmen was 10 min. Values o co osion po en ial E co and co osion cu en densi y i co we e de e mined by applying he Ta el analysis. 3. Resul s and Discussion 3.1. Mo phology o Deposi ed Ni-P Coa ings The su ace mo phology o Ni-P coa ings deposi ed on AZ91 magnesium alloy is shown in Figu e 1. The deposi ed Ni-P coa ings had high—(10.8 ± 0.1 w . % o P), me- dium—(7.4 ± 0.1 w . % o P), and low—(5.5 ± 0.1 w . % o P) phospho us con en s. All deposi ed coa ings show a nodula mo phology, which is ypical o elec oless Ni-P coa - ings [12]. As obse ed in Figu e 1a–c, he nodule size dec eases wi h inc easing P con en . As epo ed by Shu [26], he phospho us has e y low solubili y in nickel; he e o e, he educed phospho us ends o agg ega e a he bounda ies o he nickel g ains du ing dep- osi ion. Hence, he phospho us inhibi s he g ow h o Ni pa icles and pa icles o Ni-P and inc eases he numbe o nuclea ion si es. The same dependence o nodule size on phospho us con en was also obse ed by Ash iani e al. [10]. Some s uc u al de ec s and associa ed c acks we e a ely obse ed in MP Ni-P coa ings. These mic os uc u al de ec s (g ow hs, mic opo es, mic oc acks) inc eased he oughness and wo sened he co osion esis ance o MP Ni-P coa ings. Figu e 1d shows an example o a c oss-sec ion o a deposi ed LP Ni-P coa ing wi h an app oxima e hickness o 30 µm. The hickness o all he deposi ed coa ings was uni- o m ac oss each en i e c oss-sec ion and exhibi ed no ob ious de ec s o inhomogenei ies. Du ing he hea ea men , he sizes o he nodules did no change. Howe e , i was e iden ha he hea ea men led o he c acking o Ni-P coa ings (Figu e 1e–g) due o he ans o ma ion o he as-deposi ed Ni-P ma ix and he p ecipi a ion o he Ni 3 P phase. The peeling o he de o ma ion o he coa ing laye we e no obse ed. The same c acks we e obse ed in ou p e ious esea ch [27]. Figu e 1. Con . Coa ings 2021,11, 447 5 o 15 Coa ings 2021, 11, x FOR PEER REVIEW 5 o 15 Figu e 1. Su ace mo phology o Ni-P coa ings, (a) as-deposi ed low-phospho us (LP), (b) as-deposi ed medium-phos- pho us (MP), (c) as-deposi ed high-phospho us (HP), (d) c oss-sec ional mic og aph o as-deposi ed LP coa ing, (e) hea - ea ed LP, ( ) hea - ea ed MP, and (g) hea - ea ed HP. 3.2. XRD Analysis The esul s o he XRD analysis o as-deposi ed Ni-P coa ings showed c ys alline, mic oc ys alline- o-amo phous, and comple ely amo phous mic os uc u es o LP, MP, and HP coa ings, espec i ely (Figu es 2–4). The in ensi y o Ni {1 1 1} 2θ ≈ 44° o he LP Ni-P coa ing (Figu e 2) inc eased wi hin he empe a u e ange o 50-150 °C. The peak co esponding o Ni {2 0 0} o his sample was de ec ed nea he di ac ion angle o 2θ ≈ 51.5° a 200 °C. The p ecipi a ion o he Ni3P phase was obse ed a 350 °C, whe e he peak co esponding o he Ni3P di ac ion plane was de ec ed a 2θ ≈ 41.7°. The dis inc c ys alliza ion o he Ni and Ni3P phases occu ed a 400 °C, when he b oad peak o Ni ans o med o a sha p c ys alline peak and i s in- ensi y apidly inc eased. The g ow h o Ni and Ni3P c ys alli es occu ed in he empe - a u e ange om 400 o 550 °C. The c ys alli e coa sening o bo h Ni and Ni3P was ob ious a 550 °C, as indica ed by he inc ease in peak in ensi ies (Figu e 2). The p esence o NiO was e iden be ween 400 and 550 °C, due o he oxida ion o he Ni-P coa ing. The g ea es inc ease in he in ensi y o Ni3P was eco ded be ween 350 and 400 °C. Fo he MP Ni-P coa ing (Figu e 3), he inc ease in Ni {1 1 1} in ensi y indica ed i s c ys alliza ion a 250 °C. The p esence o Ni3P was de ec ed a a lowe empe a u e (330 °C) bu wi h a highe alue o in ensi y (e.g., Ni3P {3 2 1} a 2θ ≈ 41.7°) compa ed o he LP coa ing. The dis inc c ys alliza ion o he Ni and Ni3P phase was obse ed a 400 °C. The o ma ion o NiO was simul aneously obse ed a 400 °C a 2θ ≈ 63°. The in ensi y o Ni and Ni3P inc eased wi h inc easing empe a u e due o he inc ease in he c ys alli e size. F om he XRD pa e ns (Figu e 3), i is possible o de ec peaks a 2θ ≈ 32°, 34°, and 36.5° co esponding o he p ima y α-Mg phase. The α-Mg phase was also de ec ed in he wo ks o Gu [28] and Hu [29]. I s p esence can be explained by he sepa a ion o Mg alloy om he Ni-P coa ing du ing he XRD sample p epa a ion. Figu e 1. Su ace mo phology o Ni-P coa ings, ( a ) as-deposi ed low-phospho us (LP), ( b ) as-deposi ed medium-phospho us (MP), ( c ) as-deposi ed high-phospho us (HP), ( d ) c oss-sec ional mic og aph o as-deposi ed LP coa ing, ( e ) hea - ea ed LP, ( ) hea - ea ed MP, and (g) hea - ea ed HP. Figu e 1d shows an example o a c oss-sec ion o a deposi ed LP Ni-P coa ing wi h an app oxima e hickness o 30 µ m. The hickness o all he deposi ed coa ings was uni o m ac oss each en i e c oss-sec ion and exhibi ed no ob ious de ec s o inhomogenei ies. Du ing he hea ea men , he sizes o he nodules did no change. Howe e , i was e iden ha he hea ea men led o he c acking o Ni-P coa ings (Figu e 1e–g) due o he ans o ma ion o he as-deposi ed Ni-P ma ix and he p ecipi a ion o he Ni 3 P phase. The peeling o he de o ma ion o he coa ing laye we e no obse ed. The same c acks we e obse ed in ou p e ious esea ch [27]. 3.2. XRD Analysis The esul s o he XRD analysis o as-deposi ed Ni-P coa ings showed c ys alline, mic oc ys alline- o-amo phous, and comple ely amo phous mic os uc u es o LP, MP, and HP coa ings, espec i ely (Figu es 2–4). The in ensi y o Ni {1 1 1} 2 θ≈ 44 ◦ o he LP Ni-P coa ing (Figu e 2) inc eased wi hin he empe a u e ange o 50-150 ◦ C. The peak co esponding o Ni {2 0 0} o his sample was de ec ed nea he di ac ion angle o 2 θ≈ 51.5 ◦ a 200 ◦ C. The p ecipi a ion o he Ni 3 P phase was obse ed a 350 ◦ C, whe e he peak co esponding o he Ni 3 P di ac ion plane was de ec ed a 2 θ≈ 41.7 ◦ . The dis inc c ys alliza ion o he Ni and Ni 3 P phases occu ed a 400 ◦ C, when he b oad peak o Ni ans o med o a sha p c ys alline peak and i s in ensi y apidly inc eased. The g ow h o Ni and Ni 3 P c ys alli es occu ed in he empe a u e ange om 400 o 550 ◦ C. The c ys alli e coa sening o bo h Ni and Ni 3 P was ob ious a 550 ◦ C, as indica ed by he inc ease in peak in ensi ies (Figu e 2). The p esence o NiO was e iden be ween 400 and 550 ◦ C, due o he oxida ion o he Ni-P coa ing. The g ea es inc ease in he in ensi y o Ni3P was eco ded be ween 350 and 400 ◦C. Coa ings 2021,11, 447 6 o 15 Coa ings 2021, 11, x FOR PEER REVIEW 6 o 15 Figu e 2. Mic os uc u al XRD analysis o LP Ni-P coa ing. Figu e 3. Mic os uc u al XRD analysis o MP Ni-P coa ing. Figu e 2. Mic os uc u al XRD analysis o LP Ni-P coa ing. Coa ings 2021, 11, x FOR PEER REVIEW 6 o 15 Figu e 2. Mic os uc u al XRD analysis o LP Ni-P coa ing. Figu e 3. Mic os uc u al XRD analysis o MP Ni-P coa ing. Figu e 3. Mic os uc u al XRD analysis o MP Ni-P coa ing. Coa ings 2021,11, 447 7 o 15 Coa ings 2021, 11, x FOR PEER REVIEW 7 o 15 Figu e 4. Mic os uc u al XRD analysis o HP Ni-P coa ing. As shown in Figu e 4, a b oad peak 2θ ≈ 40° o 52° a 50 °C indica es he HP Ni-P coa ing o ha e a comple ely amo phous mic os uc u e. Ni c ys alliza ion was obse ed a a lowe empe a u e (300 °C) compa ed o he Ni-P coa ings wi h lowe phospho us con en . Howe e , he onse o p ecipi a ion o he Ni 3 P phase was obse ed a 320 °C. Fo LP and MP coa ings, a signi ican inc ease in he c ys alli e size o Ni and Ni 3 P occu ed be ween 400 and 550 °C. Nickel oxide NiO was again de ec ed be ween 400 and 550 °C (Figu e 4). F om he measu ed da a, i is e iden ha he empe a u e o Ni c ys alliza ion in- c eased and he empe a u e o Ni 3 P p ecipi a ion dec eased wi h inc easing P con en in he Ni-P coa ings (Figu e 5). Figu e 5. The e ec o P con en in Ni-P coa ing on Ni c ys alliza ion and Ni 3 P p ecipi a ion em- pe a u e. Figu e 4. Mic os uc u al XRD analysis o HP Ni-P coa ing. Fo he MP Ni-P coa ing (Figu e 3), he inc ease in Ni {1 1 1} in ensi y indica ed i s c ys alliza ion a 250 ◦ C. The p esence o Ni 3 P was de ec ed a a lowe empe a u e (330 ◦ C) bu wi h a highe alue o in ensi y (e.g., Ni 3 P {3 2 1} a 2 θ≈ 41.7 ◦ ) compa ed o he LP coa ing. The dis inc c ys alliza ion o he Ni and Ni 3 P phase was obse ed a 400 ◦ C. The o ma ion o NiO was simul aneously obse ed a 400 ◦ C a 2 θ≈ 63 ◦ . The in ensi y o Ni and Ni 3 P inc eased wi h inc easing empe a u e due o he inc ease in he c ys alli e size. F om he XRD pa e ns (Figu e 3), i is possible o de ec peaks a 2θ≈32◦, 34◦, and 36.5◦ co esponding o he p ima y α -Mg phase. The α -Mg phase was also de ec ed in he wo ks o Gu [ 28 ] and Hu [ 29 ]. I s p esence can be explained by he sepa a ion o Mg alloy om he Ni-P coa ing du ing he XRD sample p epa a ion. As shown in Figu e 4, a b oad peak 2 θ≈ 40 ◦ o 52 ◦ a 50 ◦ C indica es he HP Ni-P coa ing o ha e a comple ely amo phous mic os uc u e. Ni c ys alliza ion was obse ed a a lowe empe a u e (300 ◦ C) compa ed o he Ni-P coa ings wi h lowe phospho us con en . Howe e , he onse o p ecipi a ion o he Ni 3 P phase was obse ed a 320 ◦ C. Fo LP and MP coa ings, a signi ican inc ease in he c ys alli e size o Ni and Ni 3 P occu ed be ween 400 and 550 ◦ C. Nickel oxide NiO was again de ec ed be ween 400 and 550 ◦ C (Figu e 4). F om he measu ed da a, i is e iden ha he empe a u e o Ni c ys alliza ion inc eased and he empe a u e o Ni 3 P p ecipi a ion dec eased wi h inc easing P con en in he Ni-P coa ings (Figu e 5). Coa ings 2021,11, 447 8 o 15 Coa ings 2021, 11, x FOR PEER REVIEW 7 o 15 Figu e 4. Mic os uc u al XRD analysis o HP Ni-P coa ing. As shown in Figu e 4, a b oad peak 2θ ≈ 40° o 52° a 50 °C indica es he HP Ni-P coa ing o ha e a comple ely amo phous mic os uc u e. Ni c ys alliza ion was obse ed a a lowe empe a u e (300 °C) compa ed o he Ni-P coa ings wi h lowe phospho us con en . Howe e , he onse o p ecipi a ion o he Ni 3 P phase was obse ed a 320 °C. Fo LP and MP coa ings, a signi ican inc ease in he c ys alli e size o Ni and Ni 3 P occu ed be ween 400 and 550 °C. Nickel oxide NiO was again de ec ed be ween 400 and 550 °C (Figu e 4). F om he measu ed da a, i is e iden ha he empe a u e o Ni c ys alliza ion in- c eased and he empe a u e o Ni 3 P p ecipi a ion dec eased wi h inc easing P con en in he Ni-P coa ings (Figu e 5). Figu e 5. The e ec o P con en in Ni-P coa ing on Ni c ys alliza ion and Ni 3 P p ecipi a ion em- pe a u e. Figu e 5. The e ec o P con en in Ni-P coa ing on Ni c ys alliza ion and Ni 3 P p ecipi a ion empe a u e. The shi o Ni c ys alliza ion o highe empe a u es is caused by he mic os uc u e o he as-deposi ed Ni-P coa ings. LP Ni-P coa ings show a c ys alline mic os uc u e and he phospho us a oms p esen do no cause such a dis o ion o he Ni la ice [ 15 , 16 ]. The e o e, he e is no need o a la ge amoun o ene gy o a ange Ni a oms in o c ys alline Ni c ys alli es, as in he case o Ni-P coa ings wi h highe P con en , whe e he mic os uc u e is mo e dis o ed o amo phous [ 30 ]. Hence, he coa ings wi h a highe P con en equi ed mo e ene gy o a oms o ea ange hemsel es and o m Ni clus e s [ 15 , 30 , 31 ]. The lowe empe a u e o he p ecipi a ion o Ni 3 P can be explained by he p esence o highe P con en in he mic os uc u e [ 32 ]. Fo HP coa ings, he e is much mo e equen in e ac ion be ween he ee phospho us a om and h ee a oms o nickel o o m he Ni 3 P phase, when compa ed o MP and LP coa ings [11,15,16]. Some au ho s [ 16 , 30 , 33 ] epo ed ha he mic os uc u e o ms and new phases occu a e he hea ing o elec oless Ni-P coa ings. Hu [ 34 ] epo ed ha LP Ni-P coa ings wi h c ys alline o mic oc ys alline mic os uc u e ans o m di ec ly in o a mix u e o c ys alline nickel ma ix and s able Ni 3 P phase. Coa ings wi h a highe phospho us con en i s ans o m in o a mix u e o c ys alline Ni and me as able Ni 12 P 5 and Ni 5 P 2 phases a lowe empe a u es (200–300 ◦ C). Then, hese me as able phases pass o Ni 3 P when he empe a u e g adually ises. The s able Ni 3 P phase is only appa en o e a empe a u e o 400 ◦C [16,35]. Howe e , Figu es 2–4show ha he e was no o ma ion o me as able phases (Ni 12 P 5 and Ni 5 P 2 ) in he empe a u e ange o 50–350 ◦ C, and only he c ys alliza ion o Ni and Ni3P was obse ed. The dependence desc ibing Ni c ys alli e size on he empe a u e o indi idual Ni-P coa ings is displayed in Figu e 6a. As seen om Figu e 6a, he Ni c ys alliza ion empe a- u e di e s depending on he P con en , i.e., 150 ◦ C o LP, 250 ◦ C o MP, and 300 ◦ C o HP. A signi ican inc ease in Ni c ys alli e size was obse ed abo e 300 ◦ C o all samples. A e deposi ion, he Ni {1 1 1} c ys alli e size was de e mined by he Debye–Sche e me hod and ound o be 38, 23, and 21 Å o LP, MP, and HP, espec i ely. The Ni c ys alli e size du ing he hea ing p ocess up o 550 ◦ C g ew o alues o 595, 382, and 287 Å o LP, MP, and HP, espec i ely (Figu e 6a). The Ni c ys alli e size in MP and HP coa ings was lowe han he alue de e mined o he LP coa ing, which is in ag eemen wi h indings p esen ed elsewhe e [6,19,30]. Coa ings 2021,11, 447 9 o 15 Coa ings 2021, 11, x FOR PEER REVIEW 9 o 15 Figu e 6. Tempe a u e dependence o c ys alli e size o (a) Ni {1 1 1} and (b) Ni 3 P {3 2 1}. As men ioned in he li e a u e [11,26], elemen al phospho us has a e y low solubil- i y in nickel; i inhibi s he Ni g ain g ow h and inc eases he numbe o Ni nuclei. I can be assumed ha du ing he hea ea men up o ~300 °C, he g ow h o Ni c ys alli e is inhibi ed due o he p esence o P in Ni c ys alli es (Figu e 6a). In he empe a u e ange o ~300–400 °C, a signi ican inc ease o Ni c ys alli e occu s due o he mig a ion and ea angemen o P a oms om Ni c ys alli e, and he o ma ion o Ni 3 P phase. The in- c ease o Ni c ys alli es size was obse ed by XRD analysis, which can be explained by he dec ease o P in Ni c ys alli es. Kuma [22] assumes ha Ni c ys alli es a e o med om an amo phous Ni-P ma ix, and he Ni 3 P phase can o med om bo h he Ni-P ma ix, and om he Ni c ys alli es o med. The p esence o phospho us in Ni-P coa ings inc eases he numbe o Ni nuclei. This also esul s in a slowe g ow h o a la ge numbe o he Ni 3 P c ys alli es wi h em- pe a u e (Figu e 6b). 3.3. DSC Analysis All DSC cu es (Figu e 7) con ain a p ominen exo he mic peak co esponding o Ni 3 P ans o ma ion om he Ni-P ma ix [20,36]. DSC analysis con i med ha he em- pe a u e o Ni 3 P ans o ma ion shi s o lowe alues wi h inc easing P con en in he coa ing and mo e ene gy is eleased (Figu e 7a–c). The DSC cu e co esponding o he LP Ni-P coa ing (Figu e 7a) shows ha Ni 3 P ans o ma ion eached i s maximum a 398 °C and he ene gy e ol ed was calcula ed o be 347.4 mJ·mol −1 . DSC peaks o MP and HP Ni-P coa ings (Figu e 7b,c) we e obse ed a empe a u es o 389 and 355 °C, wi h co e- sponding e ol ed ene gies o 390.6 and 507.9 mJ·mol −1 , espec i ely. The empe a u e de- e mined co ela es wi h he indings om XRD (Figu e 2). Figu e 6. Tempe a u e dependence o c ys alli e size o (a) Ni {1 1 1} and (b) Ni3P {3 2 1}. The esul s also show ha wi h inc easing P con en in he Ni-P coa ing, a smalle size o Ni c ys alli e was achie ed. This is ela ed o Ni {1 1 1} di ac ion e lec ions in XRD spec a (Figu e 6). F om Figu e 6b i is e iden ha he p ecipi a ion o Ni 3 P occu s a lowe empe a u es wi h inc easing P con en . In he case o HP coa ings, p ecipi a ion occu ed a 320 ◦ C, whe eas p ecipi a ion occu ed a 350 and 330 ◦ C o LP and MP coa ings, espec i ely. The p esence o his phase was no de ec ed up o hese empe a u es. The empe a u e dependence o Ni 3 P c ys alli e size was almos linea , whe eas he g ow h a e o he Ni 3 P phase was he highes o LP Ni-P coa ings and he lowes o HP Ni-P coa ings. The Ni 3 P c ys alli e sizes {3 2 1} a 550 ◦ C we e 441, 256, and 179 Å o LP, MP and HP coa ings, espec i ely. Figu e 6b also shows ha a compa able Ni 3 P c ys alli e size (~250 Å) occu ed in he na ow empe a u e ange om 390 o 400 ◦C. As men ioned in he li e a u e [ 11 , 26 ], elemen al phospho us has a e y low solubili y in nickel; i inhibi s he Ni g ain g ow h and inc eases he numbe o Ni nuclei. I can be assumed ha du ing he hea ea men up o ~300 ◦ C, he g ow h o Ni c ys alli e is inhibi ed due o he p esence o P in Ni c ys alli es (Figu e 6a). In he empe a u e ange o ~300–400 ◦ C, a signi ican inc ease o Ni c ys alli e occu s due o he mig a ion and ea angemen o P a oms om Ni c ys alli e, and he o ma ion o Ni 3 P phase. The inc ease o Ni c ys alli es size was obse ed by XRD analysis, which can be explained by he dec ease o P in Ni c ys alli es. Kuma [ 22 ] assumes ha Ni c ys alli es a e o med om an amo phous Ni-P ma ix, and he Ni 3 P phase can o med om bo h he Ni-P ma ix, and om he Ni c ys alli es o med. The p esence o phospho us in Ni-P coa ings inc eases he numbe o Ni nuclei. This also esul s in a slowe g ow h o a la ge numbe o he Ni 3 P c ys alli es wi h empe a u e (Figu e 6b). 3.3. DSC Analysis All DSC cu es (Figu e 7) con ain a p ominen exo he mic peak co esponding o Ni 3 P ans o ma ion om he Ni-P ma ix [ 20 , 36 ]. DSC analysis con i med ha he empe a u e o Ni 3 P ans o ma ion shi s o lowe alues wi h inc easing P con en in he coa ing and mo e ene gy is eleased (Figu e 7a–c). The DSC cu e co esponding o he LP Ni-P coa ing (Figu e 7a) shows ha Ni 3 P ans o ma ion eached i s maximum a 398 ◦ C and he ene gy e ol ed was calcula ed o be 347.4 mJ · mol −1 . DSC peaks o MP and HP Ni-P coa ings (Figu e 7b,c) we e obse ed a empe a u es o 389 and 355 ◦ C, wi h co esponding e ol ed ene gies o 390.6 and 507.9 mJ · mol −1 , espec i ely. The empe a u e de e mined co ela es wi h he indings om XRD (Figu e 2).