Magnezyum alaşımlarının farklı yöntemlerle hidroksiapatit kaplanarak korozyon hızının belirlenmesi
Corrosion behaviour of ha coatings on AZ31 and AZ91 magnesium alloys
Yazar: SERKAN BAŞLAYICI
Danışman: PROF. DR. MAHMUT ERCAN AÇMA
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2021
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bu tez kapsamında, AZ31 ve AZ91 magnezyum alaşımlarının biyobozunur implant malzemesi olarak kullanılabilmesi için hidroksiapatit (HA) ile kaplanarak, in-vitro korozyon davranışları belirlenmiştir.
Son yıllarda ortopedik cerrahide kullanılan kortikal vida ve plakaların biyobozunur olarak üretilmesi üzerine çalışmalar araştırmacılar tarafından yapılmaktadır. Biyobozunur implantın avantajı, takılan implantın geri çıkarılması gerekliliğini ortadan kaldırmasıdır. Ayrıca Magnezyumun Elastisite modülünün ve mekanik özelliklerinin insan kemiğine yakın olması sonucunda, literatürde "stress shielding" olarak adlandırılan yükü sürekli olarak implantın taşıması sonucu kemiğin tembelleşmesi olayının önüne geçilmesi amaçlanmıştır.
Bu tez kapsamında, AZ31 ve AZ91 magnezyum alaşımlarının hidroksiapatit kaplanması, karakterizasyonu ve son olarak da korozyon davranışları incelenmiştir. Bu sayede biyobozunur implant malzemesi olarak kullanılabilecek Magnezyum alaşımları için en uygun kaplama koşulları belirlenmiş ve ilerleyen süreçte yapılması öngörülen in-vivo testler için optimum kaplama koşullarının neler olduğu belirlenmiştir.
Tez kapsamında uygulanan yöntemlerin ana hatları şu şekildedir: detaylı literatür taraması; hammadde ve makine-teçhizatların temini; plazma sprey kaplama çalışmaları, elektrostatik sprey kaplama çalışmaları ve daldırma kaplama çalışmaları, nitel ve nicel analizlerin gerçekleştirilmesi; kaplama yöntemlerinin karşılaştırılması, tez çıktılarının yayınlanması. Özellikle sinterleme sıcaklığı ve süresinin belirlenmesi çok önemlidir. Çünkü literatürde çok farklı parametreler denenmesine rağmen ortak bir başarılı sonuç üzerinde uzlaşılamamıştır.
Ülkemizdeki üniversiteler ve araştırma kurumlarında bu konu üzerine yapılmış herhangi bir güncel proje yer almamakta, sadece sınırlı sayıda yayın çalışması bulunmaktadır. Ayrıca uluslararası literatürde farklı yöntemlerle kaplama yaparak kıyaslama yapan çalışmalar son derece kısıtlıdır. Tez çalışmaları sonucunda elde edilen veriler ile yapılan yayın ile bilimsel ve akademik literatüre katkı sağlanmıştır. 
Bu çalışma sırasında, AZ31 ve AZ91'in hidroksiapatit kaplama işlemleri, karakterizasyonları ve korozyon davranışları incelenmiştir. Bu çalışmanın amacı, biyobozunur implant malzemesi olarak magnezyum alaşımları için optimum kaplama koşullarını belirlemektir. Yapılan çalışma sonucunda korozoyon hızını azaltma bakımından üç yöntem kıyaslandığında en iyi sonucu atmosferik plazma sprey kaplama vermiştir. En iyi sonucu veren plazma sprey kaplamanın bir sonucu olarak, korozyon hızı yaklaşık 1,2 mm / yıl'dan 0,4 mm / yıl'a düşmüştür.

Abstract:
In the scope of this thesis, in-vitro corrosion behavior of AZ31 and AZ91 magnesium alloys were determined by coating them with hydroxyapatite (HA) in order to be used as biodegradable implant material.
In recent years, studies on biodegradable production of cortical screws and plates used in orthopedic surgery have been carried out by researchers. The advantage of the biodegradable implant is that it eliminates the need to remove the implanted implant. In addition, as a result of the elasticity modulus and mechanical properties of Magnesium being close to the human bone, it is aimed to prevent the incident of laziness of the bone as a result of the implant carrying the load, which is called "stress shielding" in the literature.
Within the scope of this thesis, hydroxyapatite coating, characterization and finally corrosion behavior of AZ31 and AZ91 magnesium alloys were investigated. In this way, the most suitable coating conditions for Magnesium alloys that can be used as biodegradable implant material were determined and optimum coating conditions were obtained for in-vivo tests that were foreseen to be carried out in the future.
The use of Mg is becoming increasingly important in dental applications, cardiovascular applications, and hard tissue changes in the skeletal system. Unlike traditional bio alloys, Mg is biodegradable, its mechanical properties are very close to human bone, and Mg ions have an accelerating effect on bone repair, which is the driving force behind the idea of producing Mg-based implants. However, for Mg to be used as an implant, the corrosion rate must be reduced. It is possible to achieve this with alloying and surface modification or both.
Two of the issues to be considered when designing biodegradable implants are corrosion rate and corrosion products. If the corrosion rate is too high, the implant will degrade before it can fulfill its function. On the other hand, if the corrosion rate is too low, it will not degrade in the body at the desired time. Corrosion products should not be toxic and should not exceed the daily amounts of needed by the human. For Fe, one of the potential Biodegradable implant material candidates, the corrosion rate is less than expected is an important problem. For Zn, the fact that excess Zn intake is harmful to the human body is one of the limiting factors. On the other hand, Mg attracts the attention of researchers with its positive results in both subjects.
Titanium alloys, Co-Cr alloys and stainless steels are among the best choices as metallic bio-alloys for hard tissue replacement procedures due to some of their material properties. The mechanical properties, corrosion behavior, and bio-compatibility are the most important characteristics for these materials for this procedure. When these alloys are used in implants, a second surgery is necessary to remove these implants from bodies of patients. Therefore, development of bio-degradable materials especially for cardiovascular stents and hard tissue replacements is highly in demand in medical sector. This demand also exists in the area of ceramic based biomaterials and biopolymers. Neither ceramic based biomaterials nor the biopolymers can provide the necessary mechanical properties and biodegradability for the aforementioned procedures like bio alloys. In the last decade, lots of research has been done in this area and iron (Fe), magnesium (Mg) and zinc (Zn) are chosen as candidate materials due to their biodegradability and nontoxicity properties. The reasons for the research of magnesium alloys as biomaterials are the biocompatibility and osteointegrity properties of magnesium. The problem with magnesium is that its corrosion rate in body is too high. For instance, the desired biodegrading period for cardiovascular stents is 12-24 months but this desired value is high for magnesium. For this reason, alloying of magnesium is considered as a valuable option to reduce its corrosion rate.
Biodegradable implants act as intelligent implants and have gained more interest among the researchers in this area over the years. The main driving force behind the development of biodegradable implants is the degradability in the physiological environment. The properties of this class of materials are that they can be applied to the required area in the same way as permanent implants, but they can degrade in time without the need for a second surgery after healing has been completed.  Lifelong problems with permanent implants also diminish due to this reason. Examples of these problems include endothelial dysfunction, permanent physical discomfort, and chronic local infections. Polymers in today's medical sector are mostly used, but Mg-based, Fe-based, and Zn-based alloys have been mentioned in the literature as better biodegradable materials due to their higher strength and ductility properties. As a biodegradable implant material, Mg based alloys have significant advantages over Fe and Zn based alloys.
Hydroxyapatite is a CaP-based bioactive ceramic with a similar structure to apatite found in the human body. For a CaP compound to be named as hydroxyapatite, the molar ratio must be at least 1.67. It has hexagonal crystal structure with Ca5(PO4)3(OH) formula. It is used both in soft tissue changes and as the surface coating of metallic implants used in hard tissue changes. 
The main reasons for the use of hydroxyapatite in the surface modification of bio alloys are its biocompatibility, the surface roughness of the implants to adhere to the tissue and prevent the living body from identifying bio alloy as a foreign substance. 
There is a lot of research on the coating of traditional bio alloys 316L stainless steel and Ti alloys with hydroxyapatite and many methods are recommended. The main recommended methods can be said as Plasma spray coating, Electrophoretic Coating, Sol-gel Coating, dip coating, Electrostatic spray coating, magnetron sputter coating, Radio frequency coating. However, only a limited part of these methods are applicable for Mg alloys. Especially, the fact that the melting temperature of Mg is lower than the other two metals mentioned is a very limiting factor.
Methods such as sol-gel coating and dip coating are performed in liquid medium, the process steps are difficult and consequently difficult to apply on Mg.
Electrophoretic coating is the most common and widely used method. However, with its ease of application, plasma spray coating and electrostatic coating have recently come to the fore. 
Electrostatic spray coating is seen as a method used for coating enamel-like ceramics, especially on the steel surface.
Universities and research institutions in our country do not have any current projects on this subject, only a limited number of publication studies. In addition, studies that make comparisons by coating with different methods in the international literature are extremely limited. The publication made with the data obtained as a result of thesis studies contributed to the scientific and academic literature.
The main lines of the methods applied in the thesis are as follows: detailed literature review; supply of raw materials and machinery-equipment; plasma spray coating studies, electrostatic spray coating studies and dip coating studies, qualitative and quantitative analysis; comparison of coating methods, publication of thesis outputs. Especially determining the sintering temperature and time is very important. Because although many different parameters have been tried in the literature, a common successful result could not be reached.
During this study, hydroxyapatite coating processes, characterization and corrosion behavior of AZ31 and AZ91 were investigated. The aim of this study is to determine the optimum coating conditions for magnesium alloys as biodegradable implant material. As a result of the study, atmospheric plasma spray coating gave the best result when the three methods were compared in terms of reducing the corozoion rate. As a result of the best result plasma spray coating, the corrosion rate has dropped from about 1.2 mm / year to 0.4 mm / year.